Fire Science Basics:

 

A basic understanding of the characteristics and behavior of fire is important to anyone aspiring to be a firefighter. This section reviews key concepts relating to fire. Understanding fire behavior requires a good working knowledge of both chemistry and physics. This information is designed to give the potential firefighter candidate a good idea of what the firefighter profession is all about—the confinement, control, and extinguishment of fire.

Fire

 

Fire is a rapid, self-sustaining oxidation process generating heat and light. The components of a fire and the types of fire are discussed below.

 

Oxidation: a chemical reaction between an oxidizer and fuel.

 

Oxidizer: generally, a substance containing oxygen that will chemically react with fuel to start and/or feed a fire. Examples include oxygen in the air, fluorine gas, chlorine gas, bromine, and iodine.

 

Fuel: materials that burn. Most common fuels contain carbon, hydrogen, and oxygen. Examples include wood, paper, propane gas, methane gas, and plastics.

 

Combustion: a rapid oxidation reaction that can produce fire. The oxygen in the air (21 percent) is generally the oxidizer, chemically reacting with the fuel. All combustion reactions give off heat and are therefore exothermic reactions. Combustion is commonly called fire. If combustion is confined and a rapid pressure rise occurs, it is called an explosion.

 

 

Explosion: a rapid expansion of gases (fuel and oxygen) that have mixed prior to ignition. Common explosions encountered by firefighters are chemical and mechanical explosions. Chemical explosion: a rapid combustion reaction classified as a detonation or deflagration, depending on the rate of propagation.

 

Detonation: a reaction that propagates at the speed of sound (1,088 feet per second in air) producing a shock wave. Examples include high explosives (dynamite, blasting agents).

 

Deflagration: a reaction that propagates at less than the speed of sound. Examples include low explosives (gunpowder) and combustible gases and dusts.

 

Backdraft: an explosion caused by the sudden influx of air into an oxygen-starved area filled with a mixture of combustible gases (primarily carbon monoxide) that are heated above their ignition temperature.

 

Mechanical explosion: a physical explosion. Examples include a boiler explosion and a boiling liquid expanding vapor explosion (BLEVE).

 

BLEVE: a container failure in the form of an explosion caused by the weakening of the container shell from the heat from a fire, corrosion, or mechanical damage. If the contents inside the container are flammable, a dramatic fireball results. Pyrolysis: a decomposition reaction in a solid material, not fast enough to be self-sustaining, usually brought on by the introduction of heat. It is the precursor to combustion. Characteristics of pyrolysis include the discoloration or browning of the surface of the material and the emission of smoke vapors.

 

Exothermic reaction: a chemical reaction that generates heat. New substances formed have less heat energy than was in the reacting materials. An example is combustion.

 

Endothermic reaction: a chemical reaction causing the absorption of heat. New substances formed by the chemical reactions contain more heat energy than prior to the reaction. An example is spontaneous combustion.

 

Spontaneous combustion: an endothermic chemical reaction causing self-ignition. Examples include a pile of rags dipped in linseed oil, alkyd enamel resins, or drying oils not properly stored or discarded and wet hay inside a barn loft.

 

 

 

 

 

 

 

 

Fire triangle: a model used to help in the understanding of the three major elements necessary for ignition: heat (thermal energy), fuel, and oxidizer (oxygen). It visually depicts the ignition sequence.

 

 

 

 

Fire tetrahedron: a model that expands on the one-dimensional fire triangle. The fire tetrahedron visually shows the interrelationship among the three components of the fire triangle and further clarifies the definition of combustion by adding a fourth component (chemical chain reaction), depicting the concept of the rapid, self-sustaining oxidation reaction. The fire tetrahedron depicts the growth of ignition into a fire.

 

Heat (Thermal Energy)

 

Heat is defined as thermal energy. There are several types of heat, or thermal, energy.

 

Chemical energy: heat energy from oxidation reactions. Fire is an example.

 

Electrical energy: heat energy (resistance) developed by electrical current moving through a conductor (copper wire). Examples of electrical energy include arching, sparks, static electricity, and lightning.

 

Mechanical energy: heat energy developed from solid objects rubbing together causing friction. Mechanical heat energy is also created in the diesel engine (adiabatic process) to initiate combustion via the rapid compression of the fuel-air mixture in the cylinders.

 

Nuclear energy: heat energy released from the atom through fission (break-up of the nucleus) and fusion (combining of two or more nuclei) processes.

 

Solar energy: the heat energy of the sun in the form of rays that travel towards the Earth at the speed of light. These infrared and ultraviolet rays can be harnessed to heat homes, reflected by mirrors, and concentrated through a magnifying glass to ignite finely divided solid particles.

 

Phases of Fire

 

 

 

 

There are three phases of fire: incipient (growth), free burning (fully developed), and smoldering (decay). Each phase has its own unique characteristics and dangers to firefighters and should be understood thoroughly to enhance safety during firefighting operations inside buildings and structures. These phases are part of the standard time/temperature curve, which helps in visualizing the heat energy and temperatures attained during a fire.

 

Incipient (Growth) Phase

 

Most fires extinguished by firefighters are in this phase. In this phase, the fire is in the beginning, slow fuel combustion stage, with the oxygen content in the area still within the normal range (21 percent). There is limited heat being generated but high levels of smoke production and flammable carbon monoxide (CO) gas. Physical destruction from fire is limited to the immediate surrounding area. In certain situations, the introduction of fresh air by firefighters entering the area of fire can cause pent up CO gas to react violently and explode (backdraft), leading to serious injury while increasing the intensity of the fire. Also during this phase, there is the possibility of fire gases reaching their ignition temperatures (flashover) causing the entire area's contents to become suddenly engulfed in fire, greatly increasing the temperature of the fire and leading to the next phase of fire, the free-burning phase.

Free-Burning (Fully Developed) Phase

 

As fire spreads throughout an area, more heat and smoke are generated and travel in an upward direction toward the ceiling. During the free-burning phase, oxygen content in the area drops from 21 percent to approximately 15 percent, causing the volume of flames to eventually decrease, while smoke production continues to increase. When the oxygen level falls below 15 percent, flame generation ceases and the fire enters the next and last phase, the smoldering phase.

Smoldering (Decay) Phase

 

During this phase, the oxygen content in the area is below 15 percent, causing the rate of heat production and active flaming to decrease rapidly. Combustibles in the room have been largely consumed by the fire and are no longer actively burning. These combustibles, however, are still emitting large amounts of smoke and flammable gases. If fresh air (oxygen) is introduced into the fire area at this time, a backdraft situation is possible, since the influx of oxygen will complete the fire triangle and cause reignition of the flammable gas mixture in the area.

 

Classification of Fire

 

There are five classifications of fire based on the type of fuel involved.

Class A Fires

 

Class A fires include fires in ordinary combustibles (wood, wood products, paper, natural fibers, rubber, and plastics). Extinguishing fires in these types of materials requires water or foam or clean agents (inert gases) to absorb heat and smother the fire or dry chemical extinguishing agents (multipurpose) to inhibit the chemical chain reaction.

 

 

 

 

Class B Fires

 

Class B includes fires in animal-based, saturated fat cooking oils and greases, flammable and combustible liquids, and flammable gases. These fires need carbon dioxide to exclude air (oxygen), and dry chemical extinguishing agents, clean agents, or foam to inhibit the release of combustible vapors.

 

 

Class C Fires

 

Class C fires are fires in live electrical equipment. These fires require an extinguishing agent that is nonconductive. Water mist (safe from electric shock); dry chemical extinguishing agents; carbon dioxide, and clean agents should be used on these types of fires.

 

Note: When electrical equipment is deenergized, extinguishing agents for Class A and Class B fires may be utilized.

 

Class D Fires

 

Class D fires are fires in combustible metals or combustible metal alloys. They involve extremely hot temperatures and highly reactive fuels. Examples of combustible metals include magnesium, lithium, sodium, potassium, sodium potassium alloys, zirconium, uranium, and aluminum. There is no one type of extinguishing agent for all kinds of combustible metals. Some of the most common extinguishing agents include sodium chloride (table salt), copper-based dry powder, finely powdered graphite (preferred on lithium fires), and very dry sand. These materials must act as a heat absorbing medium as well as a smothering agent without reacting with the burning metal.

 

Class K Fires

 

Class K fires are fires in unsaturated fat vegetable cooking oils used today with more efficient cooking appliances. These oils burn hotter than animal-based, saturated fat cooking oils and remain at high temperatures longer when used with new, thermally insulated cooking and frying equipment. They require wet chemical extinguishing agents.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Heat and Temparature

 

Heat and temperature are two distinct, but closely related, concepts. Heat is a measure of the quantity of energy contained in a substance. It is the total amount of molecular vibration (energy) in a material. Temperature, on the other hand, is the average energy of its molecules. Temperature is a measure of how fast molecules are moving within a substance. It is an indicator of the level at which the heat energy exists.

Heat Units

 

Heat is measured in several ways, discussed briefly below.

British Thermal Unit

 

A British thermal unit (BTU) is the amount of heat energy required to raise the temperature of 1 lb of water (measured at 60°F at sea level) by 1° F. Common materials that burn store a standard amount of heat energy per pound. This information is valuable to firefighters when they are calculating the amount of water required during fire extinguishing operations and to fire protection engineers when they are designing and installing fire extinguishment systems and equipment.

 

One BTU is equal to 252 calories (metric heat unit), 3.96 large calories (kilogram calorie), or 1,055 joules (mechanical heat unit). Below is a list of some common combustibles and their equated latent heat of combustion:

Calorie

 

A calorie is the amount of heat energy required to raise the temperature of 1 gram of water (measured at 15 degrees Celsius [°C] at sea level) by 1° C. One calorie is equivalent to 4.184 joules.

Joule

 

The joule is the heat energy unit in the International System of Units (SI). It is the amount of heat energy provided by 1 watt flowing for 1 second.

Temperature Units

 

Temperature units can be used to compare the difference in heat energy levels between two materials. Temperature is measured by monitoring how much an object expands from its size at a given starting point (the freezing point of water, for example) and defining a unit of measurement (1 degree). All temperatures are then multiples of that defined unit of measurement.

Fahrenheit Degree

 

The Fahrenheit (F) degree is named for the German scientist Daniel Gabriel Fahrenheit, who invented the thermometer at the beginning of the eighteenth century. There are 180 increment degrees between the temperature of melting ice (32 degrees) and the boiling of water (212 degrees) on the Fahrenheit temperature scale. 1°F is equal to 5/9 degrees Celsius.

 

To convert (approximately) a temperature on the Fahrenheit scale to the Celsius or Centigrade scale, you first subtract 32 degrees from the Fahrenheit temperature and then multiply by 5/9.

 

 

 

For example, if a person's body temperature is 98.6°F, its temperature in Celsius is

 

 

Celsius Degree

 

The Celsius (C) degree is a metric unit of temperature measurement. It is named for the Swedish professor Anders Celsius, who invented the Centigrade temperature scale in the 1720s using the freezing point of water as 0 degrees and the boiling point of water as 100 degrees. This unit is approved by the SI.

 

To convert (approximately) normal body temperature on the Centigrade scale to the Fahrenheit scale, first multiply the Celsius temperature by 1.8, or 9/5, and then add 32

Rankine Degree

 

The Rankine (R) degree is a traditional unit of absolute temperature. The temperature units for Rankine and Fahrenheit are equal (1 degree Rankine represents the same temperature difference as 1 degree Fahrenheit), but the zero points differ. The zero point on the Rankine scale is set at absolute zero, which is –457.6 degrees, the hypothetical point at which all molecular movement ceases. The unit is named for British physicist and engineer William Rankine (1820–1872).

 

To convert degree units from the Rankine scale to the Fahrenheit scale and the Fahrenheit scale to the Rankine scale use the following formulas:

 

F = R – 457 and R = F + 457

Kelvin Degree

 

The Kelvin degree (K) is equal to the Celsius degree, but the Kelvin scale has its zero point set at absolute zero, which is –273.1. This unit is approved by the SI. The Kelvin degree is named for British inventor and scientist William Thompson, who was knighted by Queen Victoria in 1866 and named Baron Kelvin of Largs in 1892.

 

To convert degree units from the Kelvin scale to the Centigrade scale and the Centigrade scale to the Kelvin scale, use the following formulas:

 

C = K – 273 and K = C + 273

 

Heat Transfer

 

Heat can be transferred to other materials through conduction, convection, radiation, and direct flame contact.

 

Conduction

 

Conduction is the transfer of heat energy through a medium (usually a solid). Heat causes molecules within the material to move at a faster rate and transmit their energy to neighboring molecules. The heat of conduction can also be transferred from one material to another via direct contact in the same fashion as internal molecular movement. The amount of heat transferred and rate of travel is dependent on the thermal conductivity of the material. Dense materials (metals) are good conductors of heat energy. Fibrous materials (wood, paper, cloth) and air are poor conductors. In a fire situation, heat can be conducted via steel columns and girders to abutting wood floor joists causing them to smolder and eventually ignite.

 

Convection

 

Convection is the transfer of heat energy through a circulating medium (liquids and gases). During firefighting operations, hot air expands and rises, as do the products of incomplete combustion. Fire spread by convection is mostly in an upward and outward direction through corridors, stairwells, and shafts from floor to floor via hot air currents.

 

Radiation

 

Radiation is the transfer of heat via infrared or ultraviolet waves or rays. These heat waves travel in a straight line through space at the speed of light in all directions and are not affected by the wind. Objects exposed to radiated heat will absorb and reflect a certain amount of heat energy, depending on certain factors. The darker and duller the object, the more heat it will absorb and the greater chance it will reach its ignition temperature and burst into flames. Light-colored, shiny objects tend to reflect radiated heat, absorb less energy, and are less likely to reach their ignition temperature. Radiated heat waves will travel through space until they are absorbed by an opaque object. These waves will pass through air, glass, transparent plastics, and water. Large amounts of radiated heat can travel large distances (50–100 feet) to ignite nearby buildings and structures.

 

Direct Flame Contact

 

Direct flame contact is the transfer of heat energy via direct flame impingement or auto-exposure, such as occurs with a flame traveling upward and outward from a roof, window, or doorway to a neighboring building or exposure.

 

 

 

 

Extinguishing Agents

 

Water

 

Firefighters extinguish most fires using water. It is usually available in abundance at or near the fire. Water can be delivered onto the fire in a number of ways: hand lines stretched from the apparatus, hand lines stretched from a standpipe system located inside a building, sprinkler system, water mist system, master deluge nozzles, distributors, etc.

 

Water extinguishes fire by cooling the material (absorption of heat), smothering (steam generation), emulsification (agitation of insoluble liquids to produce a vapor inhibiting froth), and dilution (adding water to reduce the concentration of a burning soluble liquid and thereby raising its flashpoint). A review of the advantageous and disadvantageous properties of water follows.

Advantageous Properties of Water

 

There are several advantages to water as an extinguishing agent. Some of the characteristics of water that make it advantageous in extinguishing fires are listed below.

Heavy, stable liquid at ordinary temperatures.

 

High specific heat—The heat capacity is given in terms of the mass of the substance in pounds and is the amount of heat required to raise the temperature of 1 lb of a substance 1°F). All solids, liquids, and gases have specific heats. Only two liquids have higher specific heats than water: ammonia and ether. One BTU is required to raise the temperature of 1 lb of

water 1°F. To raise the temperature of 1 lb of water from 32°F to 212°F requires 180 BTU.

 

Latent heat of fusion— Melting of 1 lb of ice into water at 32°F absorbs 143.4 BTU.

 

Latent heat of vaporization— The latent heat of vaporization is the conversion of 1 lb of water into steam at a constant temperature with the absorption of 970.3 BTU.

 

Conversion to steam— The conversion of liquid water to steam increases its volume approximately 1,600 times, which displaces an equal volume of air, thereby reducing the volume of oxygen available for the oxidation reaction.

 

 

Disadvantageous Properties of Water

 

Water also has disadvantages, some of which are listed below.

It conducts electricity.

It has low viscosity, which means it runs off smoldering material readily.

It has high surface tension, since it has poor penetration qualities.

It is transparent to radiated heat.

It freezes at relatively high temperature.

It displaces flammable liquids.

It reacts violently with combustible metals.

Foam

 

Mechanical foam used today is a mixture of air, water, and liquid foam concentrate (protein- or synthetic-based). The combination of these three ingredients produces a bubble blanket solution that will flow over and around combustible solids and liquids. Unlike water, foam is lighter than flammable liquids and will float on the surface of the liquid. Foam is used on both Class A and Class B fires. It will conduct electricity, however, and should not be used on energized electrical equipment. Foam extinguishes fires in four ways: cooling; smothering; suppressing vaporization, and separating the flames from the fuel surface, thereby reducing radiated heat feedback
Carbon Dioxide

 

Carbon dioxide (CO2) is a relatively nonreactive gas that penetrates and spreads throughout the area of the fire. It is used primarily to extinguish Class B and Class C fires because it is nonconductive. CO2 extinguishes fires by displacing the air (oxygen) content that is reacting with the fuel. It also has a cooling effect on the fire, which aids in lowering the temperature of the fuel below its ignition temperature.

 

Dry Chemicals

 

The dry chemical ammonium phosphate extinguishing agent is used on Class A-, Class B-, and Class C-type fires, whereas the dry chemicals sodium bicarbonate or potassium bicarbonate are used to extinguish Class B- and Class C-type fires only. Dry chemical agents are still being used to extinguish fires in animal-based, saturated fat cooking oils. These agents leave a residue that is nonflammable on the extinguished material, which reduces the likelihood of reignition. Dry chemical agents extinguish fires by cooling, smothering, and reducing radiated heat feedback, and most importantly, by inhibiting the chemical chain reaction of the oxidation process.

 

Wet Chemicals

 

The wet chemical extinguishing agent contains a blend of potassium acetate, citrate, and water (40 to 60 percent). It is a low pH (acidic) agent that was originally developed for preengineered cooking equipment fire extinguishing systems for use on today's unsaturated fat vegetable cooking oils (Class K) that rapidly burn very hot and are difficult to extinguish. This agent is applied as a fine mist to help prevent grease splash and fire reflash while cooling the appliance.

 

Clean Agents

 

Clean agents are gases, gas mixtures, and vaporized liquids that are replacements for Halon gas extinguishing agents that are rapidly being phased out for being destructive to the Earth's ozone layer. These agents are nonconductive and noncorrosive, and they leave no residue and are therefore especially valuable in protecting telecommunication, electrical, and computerized equipment. Clean agents extinguish fire by interfering with the chemical chain reaction between oxygen and the fuel vapors, by absorbing heat, and by displacing the air (oxygen) content inside a room or space. Examples of clean agents include halocarbon gases, inert gases, powdered aerosols, and water mist. They are utilized to extinguish fires in Class A-, Class B-, and Class C-type materials.

 

Dry Powder

 

Dry powder extinguishing agents are used to extinguish fires in combustible metals and combustible metal alloy (Class D) materials. They are generally in the form of a salt (sodium chloride) or powdered copper metal and graphite-based powder. Fire in a combustible metal causes the salt to cake and form a crust over the burning material. This transformation acts to smother the metal, thereby excluding oxygen and suppressing vaporization. Salts are preferred for the extinguishment of magnesium, uranium, and powdered aluminum, sodium, potassium, and sodium/potassium alloys. Powdered copper metal and graphite-based powder are preferred for fires involving lithium and lithium alloys.

Labeling and Rating of Portable Fire Extinguishers

 

Fire extinguishers are labeled for quick identification of which classification of fire they will be effective on. The label shows both recommended and unacceptable uses for the extinguisher.

 

Also found on the label is the Underwriters Laboratories (UL) numerical rating. The system is divided into Class A and Class B ratings only. The rating system helps the user determine and compare the relative extinguishing effectiveness of portable fire extinguishers. A basic breakdown of the rating system is as follows.

 

The A rating is a water equivalent rating. The number 1 preceding the letter A is equal to 1.25 gallons of water. Numbers greater than 1 are multiplied by 1.25. For example, a 2A rating is equal to an extinguishing capacity of 2.5 gallons of water.

 

The B rating is an area of coverage equivalent rating. The number 1 preceding the letter B is equivalent to 1 square foot of coverage. Numbers greater than 1 are multiplied by 1. For example, a 20B rating is equivalent to 20 square feet of coverage.

 

Note: There is no numerical rating for Class C, Class D, and Class K fire extinguishers

 

 

Fuel (Combustible Matter—Solids, Liquids, And Gases)

 

Combustible matter may be in a solid, liquid, or gaseous state.

 

 

Solids

 

Solids are materials with defined volume, size, and shape at a given temperature. Examples are wood and wood products (paper, cardboard), carbon-containing materials (coal, charcoal), plastics (polyvinyl chloride, epoxies), textiles (cotton, wool, rayon), and combustible metals (magnesium, aluminum).

 

Wood and Wood Products

 

Wood and wood products are the most common solids encountered by firefighters. They are considered Class A-type materials and require water or water solutions to cool them below their ignition temperature and extinguish them. The average ignition temperature of wood is approximately 400°F. Major components are carbon, hydrogen, and oxygen.

 

 

Factors Affecting the Ignition and Combustibility of Wood and Wood Products

 

Many factors influence the ignition and combustibility of wood and wood products, the most important of which are cited below.

 

Physical form (size, form, shape, mass)—The greater the mass in relation to surface area, the more heat energy will be required to ignite it and the slower the rate of burning will be once ignited.

 

Thermal inertia—Resistance to heating, generally based on the specific gravity and density of the material, is known as its thermal inertia. Materials with a low thermal inertia (low specific gravity and density) will heat up and ignite more readily than materials with a high thermal inertia, high specific gravity, and density.

 

Moisture content—Wet wood is more difficult to ignite than dry wood. Wood is very difficult to ignite when the moisture content rises above 15 percent.

 

Species—Low-density softwoods (pine) will ignite at lower temperatures than high-density hardwoods (oak).

 

Ignition temperature—The minimum temperature to which a material must be heated for it to ignite and be self-sustaining without an external input of heat is known as the ignition temperature.

 

Piloted-ignition temperature—Ignition temperature caused with the assistance of an external (flame, spark) heat source is known as the piloted-ignition temperature. It is usually considerably lower than the ignition temperature.

 

Arrangement—The term arrangement refers to the spacing of the fuel material. Tightly stacked lumber is much more difficult to ignite and will burn at a slower rate than lumber loosely arranged.

 

Time—Wood and wood products must be exposed to heat for a certain period of time before combustible vapors are produced and ignite.

 

Heat Source—A heat source provides heat. For wood products heat sources include steam pipes, matches, and a blowtorch.

 

Rate of Heating—The rate or speed at which a substance becomes heated may be constant or sporadic.

 

Oxygen—Oxygen-enriched atmospheres (greater than 21 percent in air) enhance burning, whereas oxygen-deficient atmospheres (less than 15 percent in air) will generally not support combustion.

 

Carbon And Carbon Containing Materials

 

Carbon and carbon-containing materials have ignition temperatures in the range of 600° to 1,400°F, depending on the amount of carbon in them. Coal and charcoal burn hotter than wood and wood products, and they generate large quantities of toxic and flammable carbon monoxide gas. They are classified as Class A-type materials.

 

Plastics

 

Plastics are other common combustible solids, although they may be produced as a liquid or foam. Most plastics are petroleum based (hydrocarbons). They can be soft or hard and be electrically conductive or nonconductive (insulators). Manufactured plastics usually contain additives (colorants, stabilizers, lubricants), which change the chemical nature and combustibility of the original plastic. Pyrolysis doesn't occur as readily in plastics as in wood and, therefore, plastics tend to have a higher ignition temperature than wood and wood products.

 

Plastics can be divided into two categories: thermoplastics and thermosets. Thermoplastics (polypropylene, polyvinyl chloride) are formed by heat and pressure and can be reshaped repeatedly by heat and pressure. In a fire, they will melt and flow like a liquid. Thermosets (alkyds, epoxies), on the other hand, may only be formed by heat and pressure once. When subjected to the heat from a fire, they will decompose and burn. They are classified as Class A-type materials.

 

Textiles

 

Textiles include clothing, bedding, upholstery, and carpeting. In general, all textile fibers are combustible. Textiles can be divided into two categories: natural fiber and synthetic fiber.

 

Natural fiber textiles can be divided into fibers derived from plants (cotton, linen, hemp) and those derived from animals (wool, mohair, camel hair). Plant fiber is composed mostly of cellulose, which consists of carbon, hydrogen, and oxygen. During a fire situation, plant fiber will decompose and burn but will not melt. The ignition temperature of cotton is

 

 

 

 

approximately 750°F. Animal fiber, however, is chemically different from plant textile material. Protein is the major component. Animal fiber, with an ignition temperature of approximately 1,100°F, will not ignite as readily as plant fiber.

 

Synthetic fiber (rayon, nylon, polyester) is material woven from artificial fiber (plastic, hydrocarbon, metal, glass). Burning characteristics of synthetic fiber include decomposition, burning, and melting. Synthetic fiber can, however, be made flame retardant and various kinds of synthetic fiber (Nomex, Kevlar) are used in the production of "fireproof" clothing for firefighters. Synthetic fibers are classified as Class A-type materials.

 

Factors Affecting Ignition and Combustibility of Textile Products

 

Various factors affect the ignition and combustibility of textile products. Some of these are listed below.

chemical composition

weight of the fabric

type of weave

finishing treatments

 

Combustible Metals

 

The elements that will combine with oxygen, reach their ignition temperature, and burn are known as combustible metals. Metals do not, however, undergo pyrolysis to produce combustible vapors when heated. They burn on their surface with no flaming combustion. Metals that do burn produce an abundance of heat energy. When water is applied, and the water molecule separates, steam and hydrogen explosions can occur. For this reason, water, unless in large amounts, is not recommended as an extinguishing agent on combustible metals. Specific extinguishing agents (graphite, salts) have been developed to cover the surface of the burning metal and exclude oxygen. Combustible metals are classified as Class D-type materials.

 

Liquids

 

Liquids make up the stage of matter between solids and gases. A liquid has definite volume but takes the shape of the container it is being stored in. Liquids that produce vapors that burn can be divided into two categories: combustible liquids (kerosene, diesel, heavy fuel oils) and flammable liquids (gasoline, methyl alcohol, acetone). Liquids can present other hazards to firefighters besides fire (corrosiveness and toxicity). In general, liquids that burn are classified as Class B materials; however, vegetable oils used in cooking and the preparation of foods are classified as Class K materials.

 

Some key characteristics to understand concerning liquids that burn are the flash point, boiling point, specific gravity, solubility, and viscosity.

 

Flash point—The flash point is the minimum temperature of a liquid at which it emits vapors to form an ignitable mixture with air. For firefighters, the flash point is the most important property of liquids that burn. The degree of hazard will be determined by the flash point of the liquid because it is the vapors of the liquid that burn, not the liquid itself. Liquids are classified as combustible (flashpoint of 100° F or more) and flammable (flashpoint of less than 100°F).

 

Boiling point—The boiling point is the temperature of the liquid at which it will liberate the most vapors. It is the temperature at which the vapor pressure of the liquid equals atmospheric pressure. The normal boiling point of a liquid is the temperature at which it boils at sea level, usually recorded as 14.7 pounds per square inch absolute (psia). It is impossible to raise the temperature of a liquid above its boiling point, except if it is under pressure.

 

Specific gravity—The specific gravity of a liquid is the ratio of the weight of the liquid to the weight of an equal volume of water. The specific gravity of water is 1. A liquid (gasoline, 0.8) with a specific gravity less than water will float on water, whereas a liquid with a specific gravity more than 1 (sulfuric acid, 1.8) will sink.

 

Solubility—The solubility of a liquid is the percentage by weight of the liquid that will dissolve in water. The solubility of a liquid ranges from negligible (less than one tenth of 1 percent) to complete (100 percent).

 

 

 

Viscosity—Viscosity is a measure of a liquid's flow (through an opening or into a container) in relation to time. Thick liquids (molasses, asphalt, wax) are on the borderline between liquids and solids and are considered viscous.

 

Gases

 

Gases are the third stage of matter. The volume of a given amount of gas is dependent on its temperature and the surrounding pressure. An important concept for firefighters to understand regarding gases and vapors being emitted from a liquid is vapor density. Vapor density is the relative density of the gas or vapor as compared to air. The vapor density of air is 1. A gas or vapor with a vapor density more than 1 (butane, 2.1) will be heavier than air and travel along the ground surface in search of an ignition source. A gas or vapor with a vapor density less than 1 (methane, 0.55) will rise and disperse readily into the air. Gases are classified as Class B-type materials.Chemical Properties of Gases

 

Gases can be classified according to their chemical properties as flammable (burn in air), inert (will not burn in air or in any concentration of oxygen and will not support combustion), oxidizer (will not burn in air or in any concentration of oxygen but will support combustion), toxic (poisonous or irritating when inhaled), and reactive (can rearrange chemically when exposed to heat or shock and explode or can react with other materials and ignite).

 

Flammable—A gas that will burn in normal concentrations of oxygen in air is a flammable gas. When discussing flammable gases (or flammable vapors boiling off a liquid) mixing with air, the concept of flammable range must be understood. The flammable range is defined as the ratio of gas or vapor in air that is between the upper and lower flammable limits. The upper flammable limit is the maximum ratio of flammable gases/vapors above which ignition will not occur; it is too rich a mixture. The lower flammable limit is the minimum ratio of flammable gases/vapors in air below which ignition will not occur; it is too lean a mixture. Examples of flammable gases include acetylene, hydrogen, and propane.

 

Inert—An inert gas is a nonflammable gas that will not support combustion. Examples include helium, nitrogen, and argon.

 

Oxidizer—A nonflammable gas that will support combustion is known as an oxidizer. Examples include oxygen and chlorine

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Toxic—Gases that cause harm to living tissue via chemical activity are called toxic gases. They can endanger the lives and health of those who inhale or come into skin contact with it. Examples include hydrogen cyanide, carbon monoxide, and ammonia.

 

 

 

Reactive—Gases that react internally and with other materials are reactive gases. They can be heat sensitive and shock sensitive and also react with organic and inorganic substances to cause combustion. Examples include fluorine and vinyl chlori

Oxidizer

Oxygen gas is the most common oxidizer that firefighters deal with during fire operations. The atmosphere consists of 21 percent oxygen, 78 percent nitrogen, and 1 percent other elements. An oxygen-enriched atmosphere (greater than 21 percent) will enhance the rate and intensity of burning. Conversely, an oxygen-deprived atmosphere (less than 15 percent) will not be able to sustain combustion.

Chemical Chain Reaction

 

 

 

 

 

 

The chemical chain reaction process that occurs during flaming combustion is the fourth component that was added to the fire triangle model to form the fire tetrahedron model. It depicts self-sustaining combustion with an ample amount of fuel and oxygen chemically interacting. As fuel burns it generates radiant heat traveling in all directions. Heat directed back onto the burning substance (radiated feedback) helps to raise more fuel to its ignition temperature and generate more vapors to mix with air and form a combustible mixture. Additional oxygen is then drawn (entrainment) into the zone of chemical reaction. This addition of oxygen also increases the heat of burning. The chemical reaction between the components of the fuel vapors and oxygen is called an oxidation reaction. The chemical reaction we know as fire continues until all the fuel is consumed, the oxygen in a confined area is diminished, or heat dissipates beyond the zone of the chemical reaction, causing the temperature of

 

 

 

Ventilation Equipment

 

Positive Pressure Ventilation (PPV) Fans—Electric or fuel-driven fans primarily designed to provide forced, uncontaminated air into a room or building to displace the by-products of fire (smoke and toxic gases).

 

Smoke Ejectors—Fans primarily designed to eject smoke and toxic gases from an area. They have some use in drawing fresh air into an area but do not move as much air as positive pressure fans.

Salvage Equipment

 

Salvage operations require specialized equipment. The following are some of the more common types of tools/equipment used.

 

Salvage covers are generally used indoors to protect furniture, equipment, and valuables from water damage, but they may also be used as a chute to funnel water out windows and down staircases. The tarpaulins are made of canvas, waterproof-treated drop cloth material, or plastic, with reinforced edges and grommets for hanging.

 

Plastic sheeting is used to cover openings on roofs and windows to protect the interior of the structure from adverse weather conditions.

 

Dewatering pumps are electrical, engine-driven, and hydraulic-powered pumps used in conjunction with a hose line to lift and remove water from below grade areas.

 

Water vacuums are designed to suction off water from floors and carpeting. These devices consist of a vacuum nozzle and catch tank worn on the back of the firefighter.

 

 

Safety Equipment and Clothing

 

To protect their health and lives, firefighters use a variety of safety equipment while performing their duties. Some of this equipment is discussed briefly below.

 

Self-Contained Breathing Apparatus (SCBA)—Firefighters wear self-contained breathing apparatus during interior structural firefighting operations to protect against breathing toxic fumes and smoke. An SCBA is also used inside confined spaces where insufficient oxygen or poisonous vapors are present. The SCBA supplies compressed, breathable air to the wearer and includes an air cylinder, high and low pressure hoses, a regulator, and face piece. It may or may not have an integral alarm device that operates automatically should the firefighter become disabled.

 

Personal Alert Safety System (PASS) Device—A personal alert safety system is an alarm device that emits a signal when a firefighter is disabled, lost, or otherwise in distress. It can be activated both manually and automatically should the firefighter stop moving for an extended period of time.

 

Respirator—A respirator is a full-face or half-face respiratory protection device that protects the user by filtering out dust particles, organic vapors, and acid gases. The device has replaceable cartridges. The term is also used for disposable, dust/paint masks, which lack filters and do not provide protection from organic vapors and acid gas.

 

Bunker (Turnout) Gear—The structural firefighting ensemble (coat and pants) that provides flame, thermal, and mechanical (cuts, abrasions) protection is known as bunker gear. The term may also refer to the entire firefighting ensemble, including helmet, hood, and boots. The gear is made from fire-resistant fiber, synthetics, and polymer materials, such as Nomex, Kevlar, and polybenzimidazole (PBI), and generally has high-visibility striping material.

 

Reflective Vest—A reflective vest is a high-visibility garment worn over firefighting clothing. It is used by firefighters when working with and around cranes and heavy construction equipment and trucks during technical rescue and building collapse operations.

 

Fire Helmet—Constructed of fiberglass, plastic, or leather, the helmet firefighters wear provides thermal and impact head protection during firefighting operations. A long curved rear brim keeps cinders and hot runoff water off the firefighter's neck. The helmet also provides thermal ear protection (ear flaps), eye protection (eye shields), and a chinstrap to secure the helmet to the head.

 

 

Hood—Made from fire-resistant material similar to that used for manufacturing bunker gear, the hood is worn over the head to protect the areas of the head and face not covered by the helmet and face piece of the SCBA.

 

 

 

 

 

 

 

 

Fire Boots—Structural firefighting footwear, or fire boots, made from rubber and/or leather, provide thermal (Nomex/Kevlar), puncture, and impact (steel sole/shank/toe) protection of the foot, ankle, and lower leg. Some boots are designed in lengths to be pulled up over the knees for enhanced thermal protection of the lower extremities.

 

Additional Protective Gear—Includes ear muffs/plugs, safety glasses/goggles, kneepads, safety shoes, and gloves, all made of material designed to protect the firefighter. Firefighters working in confined spaces and in technical rescue work wear a hard hat. the fuel to drop below its ignition temperature.de.

 

Fire fighting equipment: Lever

 

A lever is a simple machine consisting of a bar or rigid object that is free to turn about a fixed point called the fulcrum. The fulcrum is known as the pivot point. A lever will apply an effort (force) at a different point from the resistance force (load). Levers are classified into three classes—first class, second class, and third class—based on the position of the effort (force), the resistance force (load), and the fulcrum.

First-Class Lever

 

In a first-class lever, the fulcrum is between the effort (force) and the resistance force (load). Common examples of first-class levers are the crowbar, the claw hammer (when being used to remove nails), pliers, tin snips, a car jack, and a seesaw. With a first-class lever, the direction of the force always changes. An example of this is when a downward effort (force) on the lever causes an upward movement of the resistance force (load).

 

If the fulcrum of a first-class lever is located an equal distance from the effort (force) being applied and the resistance force (load), there is no mechanical advantage—the MA is 1. The closer the fulcrum is to the load, the less effort (force) will be needed to lift the load and the mechanical advantage will be greater than 1. Remember the formula for mechanical advantage is:

 

Image Example: In the illustration shown, the load and mechanical advantage are known, and the effort needed can be calculated: Image

 

The trade off, however, is that the effort (force) will have to move (down) a greater distance and the load will move (rise up) a smaller distance.

 

Conversely, if the fulcrum is moved closer to the effort (force) being applied, greater effort (force) will be required to lift the resistance force (load) but the effort (force) will have to move a shorter distance and the resistance force (load) will move (rise up) a greater distance.

 

Note: Another mechanical principle formula that can be used to calculate the value of lever systems is the following:

 

Force × Effort Distance = Load × Resistance Distance

 

Example: A 300-pound weight is placed on the end of a plank 3 feet from a fulcrum. How much effort (force) would a firefighter have to exert on the opposite end of the plank at 9 feet from the fulcrum to obtain equilibrium?

 

Force Effort Distance = Load × Resistance Distance

 

(x) × 9 feet = 300 pounds×3 feet Divide both sides by 9 to isolate the variable. Image

Second-Class Lever

 

In a second-class lever the resistance force (load) is between the effort (force) and the fulcrum. The fulcrum will normally be closer to the load and therefore reduces the effort (force) required to accomplish the job at hand. An example of a second-class lever is the wheelbarrow. Its mechanical advantage is greater than 1. A second-class lever always enhances effort (force). Unlike the first-class lever, a second-class lever does not change the direction of effort (force). Pushing up (exerting force) on a second-class lever pushes up on the (load); conversely, pushing down on a second-class lever pushes down on the load. Other examples of second-class levers are a nutcracker, a bellows, a paper cutter, and a bottle opener.

 

 

 

 

 

 

 

 

Example: A wheelbarrow filled with building materials weighing 100 pounds that is 1.5 feet from the wheel would require how much force to lift it off the ground using handles that are 3 feet from the load? What is the mechanical advantage?

 

Force ×Effort Distance = Load × Resistance Distance

 

(x) × 3 ft = 100 lb × 1.5 ft Divide both sides by 3 ft to isolate the variable.

 

Image

 

If effort is 50 lbs to move a load of 100 pounds, the MA = 2.

Third-Class Lever

 

A third-class lever has the effort (force) between the fulcrum and the resistance force (load). In a third-class lever, the effort (force) required to lift the load is actually increased and, therefore, the mechanical advantage is less than 1. The trade-off, however, is an increase in speed and distance of travel of the load. An example of a third-class lever is a shovel. The worker's hands supply the effort (force) while the elbows act as a fulcrum. The load (soil, sand, coal) is moved at the end of the shovel. As in the second-class lever, the direction of effort (force) does not change. Examples of third-class levers are a pitchfork, tweezers, a hoe, tongs, and a broom.

 

Example: A pair of tongs is being used by fire marshals at a fire scene to pick up a one-pound circular ball to be used as evidence. Two pounds of effort force is required to lift the object. The fingers being used to squeeze the tongs are 4 inches from the object and 2 inches from the fulcrum of the tongs. What is the mechanical advantage of the tongs?

 

Simple Machines

 

A machine is any device that applies mechanical energy at a given point and delivers it in a more efficient form at another point. There are many kinds of machines with varying capabilities and functions. Specifically, machines are used-

to transform energy from one type to another (steam turbine)

to transfer energy (automobile drive train)

to increase force (pry-bar)

to multiply speed (bicycle gears)

to change the direction of force (pulleys)

to reduce friction (rollers)

 

Machines may be powered by motors, engines, or simply human effort. Prior to the age of motors and engines, animals were used to assist workers in moving and lifting heavy objects.

 

Simple machines were invented and used to overcome resistive forces and enable workers to get the job done. Firefighters have been using simple machines to perform the work they do since ancient times. Some of the simple machines they use daily include the axe, pry-bar, hook, hammer, pliers, vice-grip, shovel, crowbar, chisel, screwdriver, wheelbarrow, wedge, chock, pulley, block and tackle, hydraulic spreading and cutting tools, jacks, bulldozer, backhoe, and many, many more.

 

Simple machines are normally used when the amount of force required cannot be applied without the aid of a machine. They are also used to change the direction of effort (force) when the direction of the force to be applied is not the desired one, as in a pulley. An important point to understand is that most simple machines do not save energy; they simply allow the force required to do the work to be distributed over a longer distance or to be increased over a shorter distance. They provide a gain in effort (force) or a gain in distance, but not both. Common simple machines are the inclined plane, wedge, screw, lever, wheel and axle, pulley, and gear and belt drive.

 

More complicated (complex) machines are basically combinations of two or more simple machines. Machines that transform heat energy into mechanical energy are known as engines (steam engine and internal combustion engine). Electric motors change electrical energy into mechanical energy.

What Is Mechanical Advantage?

 

As stated above, simple machines are used to reduce the amount of force needed to perform a given task, such as moving heavy objects or lifting a load. By definition, a machine is a device that provides a mechanical advantage (MA). However, there is a trade off when simple machines are used: you must apply this force over a greater distance than if the load were moved directly. When trading off effort for distance, the advantage gained of increasing our effort force is called a mechanical advantage. The mechanical advantage of a machine is the factor by which a machine multiplies the force or effort being exerted on it. The mechanical advantage is a ratio of a load to the applied force (MA = load/applied force). A mechanical advantage greater than 1 is considered good. The greater the mechanical advantage, the less force or effort is required to accomplish the task.

How to Calculate Mechanical Advantage

 

Basic equations for finding the mechanical advantage of simple machines are:

 

Types of Simple Machines and Their Mechanical Advantage

 

Simple machines are derived from either the inclined plane or the lever. Seven common simple machines are:

The inclined plane

The wedge

The screw

The lever

The wheel and axle

The pulley

The gear and belt drive system

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

The Wheel and Axle and the Pulley

 

 

The Wheel and Axle

 

 

The wheel and axle is another type of simple machine that moves objects across distances. Wheels help move objects along the ground by decreasing the amount of friction between what is being moved and the surface. The work of this simple machine can result from the larger wheel being utilized to turn a smaller axle wheel. (Example: The steering wheel and shaft of a car enhances effort (force) with the trade-off, once again, having to apply effort (force) over a greater distance). Movement is created as the steering wheel turns, thereby applying a rotating force to its cylindrical axle post. The bigger the wheel, the greater the twisting force (torque) that can be applied to the axle.

 

Work can also result when an axle is used to rotate wheels, such as the example of a rear axle and wheels of a truck. The effort (force) is applied to the axle at a point close to where the axle turns. This can be equated as the effort (force) distance. When effort (force) is applied to the axle, the mechanical advantage will be less than one but the speed is enhanced. The distance between the point where the wheel touches the ground and the point where the wheel turns can be called the resistance force (load) distance. These two distances are equal to the radius of the axle and the radius of the wheel, respectively.

 

To calculate the mechanical advantage of a wheel and axle assembly divide the radius of the wheel by the radius of the axle.

 

Example: What is the mechanical advantage provided by a car's steering wheel assembly when the radius of the steering wheel is 6 inches and the radius of the axle is 1 inch?

 

 

 

 

 

 

 

 

 

Effort (force) is being applied to the steering wheel and therefore multiplied, providing torque on the axle six times greater than the effort (force) applied to the wheel. The trade-off, however, is that the steering wheel travels six times farther than the axle does during one full rotation.

 

Use the formula below to calculate the amount of effort (force) required when using this simple machine.

 

Effort × Circumference = Resistance Force×Circumference

 

(Force) × (Wheel) = (Load) × (Axle)

 

Example: In the drawing below of a well crank (windlass), the handle is attached to a 2-inch radius axle. The turning circumference of the crank is 16 inches. How much effort (force) is required to lift a bucket of water weighing 40 pounds?

 

 

 

 

 

 

 

The Pulley

 

A pulley can be considered as a circular lever. It is a wheel with a grooved rim and axle with a rope, belt, or chain attached to it in order to change the direction of the pull and lift a load. The effort (force) distance is the radius of the pulley (length from the axle to the side of the rope being pulled). The resistance force (load) distance is the radius of the pulley from the axle to the load-carrying side of the rope. Pulleys are used to lift heavy loads and can be found in block and tackles, cranes, hydraulic systems, and chain hoists. They change the direction of effort (force) making it easier to lift the object or they enhance the effort (force).

 

Mechanical advantage for pulley systems can be found using the following formulas:

 

 

 

OR

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Types of Pulleys

 

There are three types of pulleys: fixed, moveable, and compound. The mechanical advantage of pulley systems depends on the number of ropes, chains, etc. supporting the load. For example, using two supporting ropes to lift a resistance force (load) of 40 pounds would give you a mechanical advantage of 2.

 

fixed (Single) Pulley

 

A fixed (single) pulley is attached to a stationary object like a wall or ceiling. It acts as a first-class lever having the fulcrum at the axis and the rope acting as the bar. Fixed (single) pulleys only change effort (force) direction (you can pull down on the rope to lift the load instead of pushing up on it). They do not enhance the effort (force). Effort (force) distance equals resistance force (load) distance and, therefore, each foot of pull on the rope will lift the load one foot. It provides no mechanical advantage (MA = 1).

 

Example: The fixed (single) pulley has a resistance force (load) at one end of the rope (see diagram). The other end must have effort (force) applied downward to raise the load. The effort (force) is equal to the load in this pulley system and there is no mechanical advantage, with the MA equal to 1.

 

 

 

 

 

 

Movable Pulley

 

A movable pulley moves up and down with the effort (force). It acts as a second-class lever having the resistance force (load) between the fulcrum and the effort (force). Unlike the fixed (single) pulley, it cannot change the direction of the effort (force). Moveable pulleys, however, enhance effort (force). Their mechanical advantage is greater than 1. The trade-off is that the effort (force) distance is greater than the resistance force (load) distance.

 

The moveable pulley (see diagram) has the resistance force (load) supported by both the rope ends (the rope end attached to the upper bar and the rope end to be pulled effort [force] in the upward direction). The two upward tensions are equal and opposite in direction to the load. The mechanical advantage is 2.

 

Compound Pulley

 

A compound pulley utilizes both a fixed (single) pulley and a movable pulley. Compound pulleys provide both a change in the direction of the effort (force) as well as dramatically decreasing the effort (force) required to lift the resistance force (load). The mechanical advantage of this type of pulley is 2. The effort (force) distance, however, like with the moveable pulley, will be greater than the resistance force (load) distance.

 

 

 

Note: The mechanical advantage of pulley systems can also be calculated visually by counting the number of ropes, chains, etc. supporting the load. For example, in the illustration of the compound pulley above, there are two supporting ropes to lift the resistance force (load), giving the pulley system a mechanical advantage of 2.

 

A T of several fixed and moveable pulleys is known as a block and tackle. Archimedes showed that by using multiple pulleys, a large ship fully loaded with men could be pulled by a single man's effort.

 

Efficiency of a Pulley System

 

To calculate the efficiency of a pulley system, first determine the mechanical advantage. Next, determine the velocity ratio by dividing the distance moved by effort (force) by the distance moved by the resistance force (load). Finally, divide the mechanical advantage by the velocity ratio and multiply this number by 100 percent.

 

Example: A pulley system can lift an object weighing 50 N with an effort (force) of 10 N. The input distance is 5 m and the output distance is 0.5 m. What is the efficiency of the pulley system?

 

 

Inclined plane

The inclined plane—a slanted surface raised at one end—is a simple machine that does not move. It is used to lift heavy loads by providing the worker with a mechanical advantage. The inclined plane provides for less effort, not for less work, to lift the object. Getting heavy boxes or barrels onto a loading dock is much easier if you slide the objects up a ramp rather than lift them up. The trade-off is the greater distance to travel. Stairs and ramps provide examples of inclined planes.

 

The mathematical formula for the mechanical advantage of an inclined plane is the length of the inclined plane—the effort (force) distance—divided by the height—the resistance force (load) distance. The length of the inclined plane can never be less than the height; therefore, the MA can never be less than 1.

 

 

Example: A ramp 20 feet in length is 5 feet in height. What is the mechanical advantage of this simple machine?

 

 

The trade-off is that the length of the slope (effort distance) is 4 times greater than the resistance force (load) distance, or height of the slope.

 

Example: Select the inclined plane diagram below that gives the best mechanical advantage in lifting a heavy barrel to the height of the platform.

 

 

The correct choice is answer B. Using the MA formula for the four inclined planes shown reveals that choice B has the greatest MA, 3, compared to 1.25 for A, 2 for C, and l.5 for D. However, B would require the longest effort (force) distance, 12 feet.

 

Example: Determine the amount of force, or effort, required to move a 400-pound load to a height of 3 feet, using a 12-foot ramp.

 

 

Use the following formula:

 

Effort × Effort Distance = Resistance Force × Resistance Distance

 

OR

 

Force Length of Inclined Plane = Load × Height

 

To solve: effort (force) × 12 = 400 pounds 3 feet

 

 

The inclined plane with a MA of 4 allows a 400-pound object to be raised 3 feet with a 100-pound force.

 

Wedge and screw

 

The Wedge

 

A wedge is an inclined plane that tapers to a sharp edge. It is used to increase force. Double wedges are made up of two inclined planes and are used to split (e.g., fireplace logs), fasten, or cut. However, a wedge can also be one sloping surface (single wedge) such as that used by firefighters as a chock or doorstop. When used for cutting, the longer and thinner the wedge, the less effort (force) is required to overcome resistance. You can enhance the mechanical advantage of an axe, a type of double wedge, by sharpening it. Effort (force) is applied to the thicker edge of the wedge and is transferred to the thinner end. The wedge is also used to change the direction of force. When force is applied downward on a double wedge, it will push out in two directions helping to push things apart at right angles. Unlike the inclined plane, the wedge moves. For example, when a wedge is used to split a log, it is the log that remains in place while the wedge moves through it.

 

Many common objects are double wedges. Nails, for example, are wedges used to fasten objects together. The tip of a slotted screwdriver is a simple wedge. Knives, axe heads, chisels, and scissors are sharpened double wedges used for cutting.

 

 

To ascertain the mechanical advantage of a single wedge when it is used as a chock, simply use the same formula as that for the inclined plane, as shown below:

To ascertain the mechanical advantage for a double wedge use the formula:

 

 

Example: What is the mechanical advantage of a single wedge with a length of slope of 18 cm and height (thickness) of 6 cm being used to chock open a door by firefighters stretching hose line?

 

 

 

 

Example: Which double wedge, A or B, being used to split logs, has the greater mechanical advantage?

 

 

MA for double wedge A = 15 cm/5 cm = 3

 

MA for double wedge B = 15 cm/3 cm = 5

 

Double wedge B has the greater mechanical advantage.

 

The Screw

 

 

A screw is a simple machine similar to an inclined plane or a wedge, but in a screw the incline wraps around a shaft. A screw converts rotational motion to linear motion. The pitch of a screw is the distance between its threads; this is the distance the screw will advance during one complete rotation.

 

To find the mechanical advantage that a screw provides, you have to know the pitch. The mechanical advantage of a screw is calculated by dividing the circumference of the screwdriver handle by the pitch of the screw.

 

 

OR

 

 

The circumference of the screwdriver handle can be considered the effort distance and the pitch of the screw can be considered the resistance force.

 

Example: A screw with 10 threads per inch is being turned by a screwdriver that has a handle with a radius (r) of 1 inch. What is the mechanical advantage? To determine the mechanical advantage (MA), first calculate the circumference of the handle of the screwdriver.

 

C (circumference) = 2πr

 

C = 2 (3.14)(1 inch)

 

C = 6.28 inches

 

When using a screwdriver, the mechanical advantage of a screw is calculated as follows:

 

 

 

 

 

Hand tools

Hammers

 

A nail hammer is among the most widely used hand tools. It is used for striking and removing nails. Hammers are made in two general patterns: straight claw (ripping) and curved claw. Handles are made of wood, steel, or fiberglass. A hammer blow should always be struck squarely with the striking face of the hammer parallel to the object being struck. When striking another tool (chisel, wedge, hand punch, etc.), the hammer's striking face should have a diameter larger than the struck face of the other tool.

 

In addition to the basic straight and curved claw hammers, there are hammers designed for specialized purposes, such as light-duty tack hammers designed for driving small nails; round ball-shaped ball-peen hammers for riveting and shaping unhardened metal; soft-face hammers, or mallets, made from wood or rubber and designed for delivering blows to objects that would mar if struck with a metal hammer, and drywall hammers designed to dimple drywall prior to nailing.

 

 

Nails

 

Nails are made from wire. They range in length from 2 d (1 inch) to 60 d (6 inches). The abbreviation "d" is used for the term "penny" and is derived from the first letter of the Roman coin denarius. The common 16 d penny nail is 3 1/2 inches long and is used to fasten structural building elements together. Other types of nails include concrete, drywall, finishing, ring, roofing, shingle, and spiral.\

 

Chisels

 

Various types of chisels are used for cutting, shaping, and trimming different materials. A cold chisel made from steel is used for cutting and shaping metals such as cast iron, bronze, and copper. A wood chisel is designed for rough work on wooden materials. A masonry chisel is used with a hand drilling hammer to score or trim brick or block.

 

Pliers

 

Pliers are hand tools used to grip, turn, pull, or crimp a large variety of objects. Pliers direct the power of the handgrip into a precision grip. The long handles in relation to the nose of the pliers act as levers, enhancing the force in the hand's grip to the object being acted upon.

 

The variety of pliers today exceeds most, if not all, other types of hand tools. Linesman pliers with a side cutting feature bend lightweight metal and sever wire. Wire stripping (electrician) pliers sever and remove insulation on electrical wire without damaging the wire. Long (needle)-nose pliers are used to grip and shape lightweight metal; the slim head design facilitates crimping wires in confined, narrow spaces. Lockjaw (vise grip) pliers are basically a handheld vise that allow for the purchase on an object to be locked in and tightened prior to applying force. They are used to firmly grip lightweight metal or remove round key-lock door cylinders.

 

Wrenches

 

Wrenches are used for tightening and loosening nuts, bolts, pipes, and many other objects that are hard to turn. The tool works as a lever. The mouth of the wrench is used for gripping the object to be turned. The wrench is pulled at a right angle to the axes of the lever action and the turned object. Wrenches can be nonadjustable or adjustable to fit better around objects of various sizes that need turning. An open-end wrench is a one-piece wrench with a smooth, U-shaped opening(s). It is designed to grip two opposite faces of a bolt or nut from the side. This is advantageous in areas that are difficult to access or are obstructed.

 

A box-end wrench is a one-piece wrench with recessed, grooved, enclosed opening(s). It is often designed double-ended with different-sized box ends. The enclosed opening grips all the faces of the bolt or nut providing more torque than the open-end wrench without slipping or stripping the bolt or nut. A combination wrench is a double-ended tool with one end open and the other end enclosed. Both ends generally fit the same size bolt or nut.

 

An offset wrench is designed to provide access to obstructed bolts and nuts in recessed areas. It allows for hand clearance when turning an object flush with a work surface. An adjustable-end (crescent) wrench is an open-ended wrench with smooth, adjustable jaws used to turn bolts or nuts. A pipe, or Stillson, wrench is an adjustable wrench having serrated jaws for gripping soft iron pipe and pipe fittings. A hex key (Allen) wrench is an L-shaped, six-sided wrench used to turn machined setscrews or bolt heads designed with a hexagonal recess. And, finally, the well-known monkey wrench (named for its inventor, Charles Moncky—I kid you not!) is an old-type adjustable wrench with smooth jaws that is used for turning bolts or nuts.

 

 

A ratchet box, designed for turning bolts or nuts, has a mechanism that eliminates the need to readjust the wrench during the return stroke. A socket with ratchet handle is a hollow cylinder (socket) that fits over a bolt or nut head, used in conjunction with a drive tool (ratchet handle). Sockets are generally sold in sets of various sizes.

 

 

Screwdrivers

 

The simple handheld screwdriver is designed to tighten or loosen and remove screws. It consists of a tip or head at the end of an axial shaft that is encased inside a cylindrical handle. The handle allows the shaft to be rotated, thereby applying torque at the tip. Screwdrivers are made in a wide variety of sizes to match different screw sizes. Screwdriver heads come in many types, the most common of which are mentioned below.

 

A slot-head screwdriver is a flat-bladed screwdriver that fits a single slot screw. A Phillips head screwdriver is a cross-headed screwdriver with rounded corners. It is designed to slip off the screw when under high torque to prevent over-tightening. A hex head (Allen) screwdriver has a six-sided head and is used as an alternative tool to the hex key wrench.

 

An offset screwdriver is used to access hard-to-reach and obstructed screws where a straight shaft screwdriver is inappropriate. A ratchet screwdriver is designed for high-speed turning using a ratchet handle.

 

 

Saws

 

The key component of the saw is a blade with a cutting edge. Other components include the heel, the end closest to the handle; the toe, the end farthest from the handle; and the front, or bottom edge; and back, or top edge.

 

There are many different types of saws, designed for specific uses. The most common saws are mentioned below.

 

A crosscut saw is designed for making cuts in lumber perpendicular (at a right angle) to the grain. The cutting edge of the blade is beveled, allowing the blade to act like a knife edge and slice through the wood. A ripsaw is designed for cutting lumber parallel to the grain. The saw teeth are substantially steeper than those in a crosscut saw and have flat front edges that act as chisels. Both of these saws (like most Western saws) cut as they are pushed through the wood; unlike Japanese-type saws that cut on the pull stroke.

 

A backsaw is a thin-bladed saw having a reinforced steel or brass back that is thicker than the blade itself and limits the depth of the cut. The teeth of the blade are closely spaced. A miter saw is a back or metal-framed saw with replaceable blades. It is designed to make crosscuts and used with a miter box to make precise angle cuts.

 

A hacksaw is a fine-toothed saw with the blade under tension inside a frame. It is designed to cut metal. Finally, a coping (jigsaw) is designed to cut intricate shapes in wood. It has a thin blade tensioned inside a metal frame.

 

 

Bars

 

Bars are made of metal and come in a variety of shapes and sizes. They are generally used as first- and second-class levers for general demolition purposes (tearing out walls, removing moldings, prying apart objects, and removing nails), but are also used for chiseling, scraping, opening up wooden boxes or crates, and lifting.

 

A crowbar (wrecking bar) is a long-handled, relatively heavy metal tool with one curved end (claw) for prying and removing nails and one flattened end.

 

A pry bar is a long-handled metal tool, generally smaller and lighter than the crowbar. It has one flattened end and one tapered, pointed end and is used for prying, lifting, and removing nails. A combination pry bar and chisel tool, an offset rip bar, is similar in appearance to the crowbar. It has a small claw end and wide chisel end in an offset pattern for difficult-to-reach objects. A utility pry bar, a small, lightweight metal tool having beveled cutting edges at both ends, is designed for pulling nails, prying light objects, and scraping. A cat's paw is a small-handle pry bar with one flattened end and one rounded, curved end for prying and removing nails.

 

A nail puller is a metal bar designed to easily slide under the head or into the shank of an embedded nail for easy removal. The other end is a striking head for a hammer.

 

 

Other Common Hand Tools

 

Other common hand tools include hand punches designed to mark metal and other surfaces softer than the punch itself and to align holes and drive or remove pins and rivets; drift pins used for aligning holes in metal; and star drills used, along with a hand drilling hammer, to drill holes in masonry.

 

 

 

Screwdrivers

 

The simple handheld screwdriver is designed to tighten or loosen and remove screws. It consists of a tip or head at the end of an axial shaft that is encased inside a cylindrical handle. The handle allows the shaft to be rotated, thereby applying torque at the tip. Screwdrivers are made in a wide variety of sizes to match different screw sizes. Screwdriver heads come in many types, the most common of which are mentioned below.

 

A slot-head screwdriver is a flat-bladed screwdriver that fits a single slot screw. A Phillips head screwdriver is a cross-headed screwdriver with rounded corners. It is designed to slip off the screw when under high torque to prevent over-tightening. A hex head (Allen) screwdriver has a six-sided head and is used as an alternative tool to the hex key wrench.

 

An offset screwdriver is used to access hard-to-reach and obstructed screws where a straight shaft screwdriver is inappropriate. A ratchet screwdriver is designed for high-speed turning using a ratchet handle.

 

Saws

 

The key component of the saw is a blade with a cutting edge. Other components include the heel, the end closest to the handle; the toe, the end farthest from the handle; and the front, or bottom edge; and back, or top edge.

 

There are many different types of saws, designed for specific uses. The most common saws are mentioned below.

 

A crosscut saw is designed for making cuts in lumber perpendicular (at a right angle) to the grain. The cutting edge of the blade is beveled, allowing the blade to act like a knife edge and slice through the wood. A ripsaw is designed for cutting lumber parallel to the grain. The saw teeth are substantially steeper than those in a crosscut saw and have flat front edges that act as chisels. Both of these saws (like most Western saws) cut as they are pushed through the wood; unlike Japanese-type saws that cut on the pull stroke.

 

A backsaw is a thin-bladed saw having a reinforced steel or brass back that is thicker than the blade itself and limits the depth of the cut. The teeth of the blade are closely spaced. A miter saw is a back or metal-framed saw with replaceable blades. It is designed to make crosscuts and used with a miter box to make precise angle cuts.

 

A hacksaw is a fine-toothed saw with the blade under tension inside a frame. It is designed to cut metal. Finally, a coping (jigsaw) is designed to cut intricate shapes in wood. It has a thin blade tensioned inside a metal frame.

 

Bars

 

Bars are made of metal and come in a variety of shapes and sizes. They are generally used as first- and second-class levers for general demolition purposes (tearing out walls, removing moldings, prying apart objects, and removing nails), but are also used for chiseling, scraping, opening up wooden boxes or crates, and lifting.

 

A crowbar (wrecking bar) is a long-handled, relatively heavy metal tool with one curved end (claw) for prying and removing nails and one flattened end.

 

A pry bar is a long-handled metal tool, generally smaller and lighter than the crowbar. It has one flattened end and one tapered, pointed end and is used for prying, lifting, and removing nails. A combination pry bar and chisel tool, an offset rip bar, is similar in appearance to the crowbar. It has a small claw end and wide chisel end in an offset pattern for difficult-to-reach objects. A utility pry bar, a small, lightweight metal tool having beveled cutting edges at both ends, is designed for pulling nails, prying light objects, and scraping. A cat's paw is a small-handle pry bar with one flattened end and one rounded, curved end for prying and removing nails.

 

A nail puller is a metal bar designed to easily slide under the head or into the shank of an embedded nail for easy removal. The other end is a striking head for a hammer.

 

Other Common Hand Tools

 

Other common hand tools include hand punches designed to mark metal and other surfaces softer than the punch itself and to align holes and drive or remove pins and rivets; drift pins used for aligning holes in metal; and star drills used, along with a hand drilling hammer, to drill holes in masonry.

 

 

Probability and Combinations

Probability

 

Probability attempts to quantify the notion of probable. The probability of an event occurring, P(E), is generally represented as a real number between 0 and 1. The more likely the probability of an event occurring the closer the probability is to 1. It should be understood, however, that a probability of 0 is not impossible, nor a probability of 1 a certainty.

 

Note: If P(E) is the probability that an event will occur, then P(E) cannot be a negative number.

 

Example: A coin is thrown in the air five times. If the coin lands with the tail up on the first four tosses, what is the probability that the coin will land with the tail up on the fifth toss?

 

Answer: The fifth toss is independent of the first four tosses and therefore the probability remains one out of two.

Combinations and the Counting Principle

 

For any word problem that involves two or more actions or objects, each having a number of choices, and asks for the number of combinations, use the Counting Principle formula:

 

Number of ways = x · y

 

Example: A firefighter, during her meal period, wants a sandwich and a seltzer from the corner delicatessen. If the deli has 6 choices of sandwiches and 4 choices of seltzer to choose from, how many different ways can she order her lunch?

 

Number of ways = (6 sandwiches) (4 seltzers)

 

Number of ways = 24

 

Example: How many different 5-letter arrangements can be formed using the letters FIRES, if each letter is used only once?

 

Five letters can be used to fill the first position in the new word, then four letters to fill the second position, three for the third position and so on. Therefore, the number of ways is: 5 × 4 × 3 × 2 × 1 = 120.

 

 

 

 

 

 

 

Gear and Belt Drive System

 

Gears are toothed wheels that are meshed together to transmit a twisting force (torque) and motion. They are usually attached to a shaft and can be considered a rotating lever. Utilizing leverage principles, gears can enhance or inhibit effort (force) or change effort (force) direction. Gears are either turned by a shaft or they turn the shaft. A large gear can apply more twisting force on a shaft to which it is attached than a smaller gear.

 

Gears that have straight teeth (perpendicular to their facing) and that mesh together in the same plane with axles parallel are known as spur gears. Spur gears provide an important way of transmitting a positive motion between two shafts. They give a smooth and uniform drive.

 

In the diagram, one gear, labeled the driver (also known as the driving gear) is turned by a motor. As it turns, it turns the other gear, known as the driven gear. A basic rule concerning gears states that each gear in a series of gears reverses the direction of rotation of the previous gear.

 

 

Another basic rule of gears is that when you have a pair of meshing gears, and the smaller gear with less pitch diameter (number of teeth) is the driver, torque output will be enhanced, with the trade-off being a decrease in speed of rotation. Conversely, when a larger gear with greater pitch diameter is the driver, torque output will be inhibited, but the trade-off is the speed of rotation will be enhanced.

 

Example: The driving (driver) gear has 9 teeth while the driven gear has 36 teeth. Find the gear ratio and mechanical advantage (torque) of this gear system.

 

 

 

Note: When computing gear ratio, always compare the larger gear rotating once to the smaller gear regardless of whether it is a driver or driven gear.

 

Gear ratio formula:

 

Input movement (Driver): Output movement (Driven)

 

1: 4

 

OR

 

 

The driver gear rotates 4 times faster than the driven gear (decrease in speed).

 

Mechanical advantage formula:

 

 

Example: The driving (driver) gear has 90 teeth while the driven gear has 15 teeth. Find the gear ratio and mechanical advantage (torque) of this gear system. Gear ratio formula:

 

Input movement (Driver): Output movement (Driven)

 

1: 6

 

OR

 

 

 

The driver gear rotates 6 times slower than the driven gear (increase in speed).

 

Mechanical advantage formula:

 

Revolutions Per Minute

 

As a general rule, if the number of revolutions per minute (rpm) of the driver gear is given and the driver gear is smaller than the driven gear, divide the gear ratio number of the driver gear into the rpm of the smaller gear (driver).

 

Example: In a two-gear system, the driver gear with 25 teeth revolves at 60 rpm; what is the rpm for the driven gear with 75 teeth?

 

Gear ratio formula:

 

Input movement (Driver): Output movement (Driven)

 

3: 1

 

OR

 

 

The driver gear rotates 3 times faster than the driven gear (decrease in speed).

 

Divide the gear ratio number of the driver into the rpm for the driver gear.

 

(60 rpm/3) = 20 rpm (agrees with driver gear rotating 3 times faster than the driven gear)

 

This adheres to the general rule of gears, which states that when a smaller gear drives a larger gear the speed should decrease.

 

Another general rule is that if the driver rpm is given and the driver gear is larger than the driven gear, multiply the gear ratio number of the driven gear by the rpm of the larger gear (driver).

 

Example: In a two-gear system, the driver gear with 60 teeth revolves at 120 rpm; what is the rpm for the driven gear with 30 teeth?

 

Gear ratio formula:

 

Input movement (Driver): Output movement (Driven)

 

2: 1

 

OR

 

 

The driver gear rotates 2 times slower than the driven gear (increase in speed).

 

Multiply the gear ratio number of the driven gear by the driver gear's rpm.

 

2 × 120 rpm = 240 rpm (agrees with driver gear 2 times slower than the driven gear)

 

This adheres to the general rule of gears, which states that when a larger gear drives a smaller gear the speed should increase.

 

Gear Trains

 

When solving questions using more than two gears (gear trains) concentrate on two gears at a time. Give each gear a letter designation. Focus on the driver gear and the direction in which it is rotating (clockwise or counterclockwise). Gears on either end of the driver gear will rotate in the opposite direction. These gears in turn will cause the gears abutting them to rotate in opposite directions. A gear train component used to allow adjacent gears to rotate in a desired direction is known as an idler gear.

 

In the illustration shown, the driver (gear B) rotates in a clockwise direction. Gears A and C, abutting gear B, rotate in a counterclockwise direction. Gears D and E, which are abutting gear C, rotate in a clockwise direction.

 

Pedal and Sprocket Gears

 

Special gears with elongated teeth are connected with a chain. These gears (pedal and sprocket) are found quite commonly in bicycles. They operate the same way as gears except that the direction of the rotating gears is not reversed. When the large pedal gear toward the front of the bicycle revolves, the chain pulls round the sprocket gear wheel at the rear in the same direction. Gear ratios and mechanical advantage are calculated via the same formulas used for gear systems.

    

Example: If the pedal (driver) gear with 30 teeth in the illustration below revolves once, how many times will the sprocket (driven) gear with 15 teeth revolve? Find the gear ratio as well as the mechanical advantage of this gear system.

 

 

 

Gear ratio formula:

 

Input movement (Driver): Output movement (Driven)

 

1: 2

 

OR

 

 

The pedal (driver) gear is 2 times slower than the sprocket (driven) gear (increase in speed).

 

Mechanical advantage formula:

 

Belt Drive

 

In machinery, a pair of pulley wheels attached to parallel shafts and connected by a flexible band of flat leather, rubber, or similar material is known as a belt drive. Like gear trains, a belt drive can be used to increase or reduce the speed and mechanical advantage (torque) of the pulley wheels they are attached to and modify rotational motion from one shaft to the other. Unlike gears, however, belt drives rotate in the same direction. A belt's top surface can also be used to convey materials across it, such as on a conveyor belt. Belts are installed under tension in order to create friction, which allows the belt to grip and turn the pulley wheels. Substantial tension also keeps the belt from slipping off the pulley wheels when rotating. Belt drives are better for greater distances with smaller forces since they are not directly joined.

 

There are two types of belt drives, positive and nonpositive. Positive belt drives (gear belts) consist of belts with teeth that mesh with pulley wheels that also have teeth. This type of belt drive does not allow the belt to slip. They are used in applications requiring a higher horsepower or torque capacity. A nonpositive belt drive system utilizes smooth belts and pulley wheels. The design of a nonpositive belt is in the shape of a "V.' These belts require less tension than do flat belts because they have more surface area in contact with the pulley wheels. Nonpositive belt drives are useful for connecting shafts that are in close proximity to one another. The V-belt found inside of an automobile engine is an example of the nonpositive type. Crossed (twisted) belts cause shafts to rotate in opposite directions.

 

The mechanical principles and applications that apply to gears, in general, also apply to belt drives. To calculate the mechanical advantage of a belt drive system, instead of counting the teeth as you would when working with gears, divide the diameter of the driven pulley wheel by the diameter of the driver pulley wheel. Whenever the driven pulley wheel diameter is larger than the driver pulley wheel diameter, you will obtain a mechanical advantage.

 

 

Example: A belt drive system has a 9-inch diameter driven pulley wheel and a 3-inch diameter driver pulley wheel. What is the mechanical advantage of the system?

 

 

 

To calculate the speed of the belt drive pulley system above, which has a driver pulley speed of 1200 rpm, use the following formula:

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Mechanical Aptitude

 

Mechanical aptitude is tested on many firefighter exams. This chapter will familiarize you with commonly tested concepts by presenting definitions, study tips, and sample test questions for basic mechanical devices and systems.

 

Firefighters use mechanical devices every day: simple hand tools such as axes and wrenches, as well as more complex systems such as pumps and internal combustion engines. The ability to understand and use mechanical concepts is critical to a firefighter's job.

 

If your exam includes a section on mechanical aptitude, it may cover topics with which you are very familiar, as well as some that are new. Regardless of your background, understanding the concepts in this chapter will benefit you both during the exam and in your career as a firefighter. After an introduction to mechanical aptitude questions, this chapter summarizes some of the most commonly tested mechanical devices and mechanical systems. It also suggests ways in which you can further improve your knowledge of mechanical devices and related scientific and mathematical knowledge. Finally, it gives you an opportunity to review what you have learned by presenting a sample mechanical aptitude section like those found on firefighter exams.

What Mechanical Aptitude Questions Are Like

 

Mechanical aptitude questions tend to cover a wide range of topics. The questions will usually be multiple choice with four or five possible answers. Some questions may require previous knowledge of the topic—so it is a good idea to study this chapter well! Other questions will include all of the information you will need.

 

Some questions will require the identification of various mechanical tools or devices. Some of the types of mechanical devices that may appear on the exam—and covered in this chapter—include hand tools, gears, pulleys, levers, fasteners, springs, valves, gauges, and pumps. In addition to individual mechanical devices, the exam may test your knowledge of various systems, or combinations of mechanical devices. A common example of a mechanical system is the internal combustion engine of an automobile.

 

A typical mechanical aptitude question will look something like this:

Which of the following is a common component of an internal combustion engine?

a piston

a compass

a hammer

a hydraulic jack

 

The answer is a, a piston. A compass is used to determine a direction on a map. A hammer is used to drive nails. A hydraulic jack is used to lift heavy items.

What Is a Mechanical Device?

 

A mechanical device is a tool designed to make a given task easier. For example, you could drive a nail into a piece of wood with a rock. However, a long time ago, someone who spent a lot of time building things with wood figured out that it would be a lot more efficient to use something that was easier to hold on to than a rock. He or she thought that a long slender handle might be nice, and that a hard piece of metal for striking the nail would provide more accuracy and not damage the wood as easily. Thus, the hammer was born.

 

Most mechanical devices were invented in the same manner: People looking for easier ways to perform their everyday jobs. Some mechanical devices are thousands of years old, such as the lever, the wheel, and many hand tools. Other more complex devices, such as pumps and valves, were invented more recently. Many times, the idea of a new mechanical device exists but the technology to make it does not. For example, many years before the pump was invented, people probably discussed the need for an easier way to move water from the river to the town on the hill. However, the technology for casting metal had not yet been invented, so the pump could not possibly have been invented at that time.

 

Mechanical devices cover a wide range of types of tools. In general, they are tools that relate to physical work and are governed by mechanical forces and movements. You can usually see what they do and how they work—as opposed to, say, a light switch or a battery, which are electrical devices. Some tools are used to directly accomplish a specific task, as when you use a hand saw to cut a piece of wood. Others, such as pulleys and gears, may be used indirectly to accomplish certain tasks that would be possible without the device but are easier with it. Still others, such as gauges, only provide feedback information on the operation of other mechanical devices. You see and use mechanical devices many times each day, so there is no reason to be intimidated by a mechanical aptitude section on the exam.

Commonly Tested Mechanical Devices

 

The following sections review some of the mechanical devices that are most likely to appear on firefighter exams.

Hand Tools

 

Hand tools are defined as tools operated not by motors but rather by human power. There are many different types of hand tools, including carpentry tools, automotive hand tools, and hand tools used specifically by firefighters. This chapter cannot cover every conceivable hand tool, so it will be limited to tools used in everyday situations and those specific to firefighting—the ones you are most likely to be tested on.

 

Some of the hand tools used by carpenters and other workers, including firefighters, are listed in the table below, along with their most common uses and some examples of each kind.

 

Hand tools used in the fire and rescue service are often classified by their main function or their size. In general, a large hand tool would not fit in a standard tool box, whereas a small hand tool would. Most fire apparatus carries a supply of standard, everyday small hand tools such as wrenches, screwdrivers, and hammers. These tools are used for a variety of purposes. For example, if a fire occurs in an electrical breaker box, firefighters will need wrenches or screwdrivers to disassemble the box to check if the fire is out.

 

Many common larger hand tools have uses in the fire service. Automotive jacks and high-lift jacks are used in vehicle rescue to stabilize cars and trucks. Sledgehammers are used in forcible entry as well as in other applications. Tools such as axes and ladders have obvious uses in the fire service, although the versions of these tools used for fire service are typically heavier or have greater capacity than domestic models. For example, fire service ladders have much greater capacity than a ladder you may have in your home, and are much heavier.

 

Specialty tools are often classified by the main function they serve. Some examples are pulling tools such as hooks or pike poles, and prying tools such as pry bars. Some tools also have secondary uses. An axe is normally used for cutting; however, a flat-head axe can also be used as a striking tool to drive another tool.

Gears

 

A gear is generally a toothed wheel or cylinder that meshes with another toothed element to transmit motion or to change speed or direction. Gears are typically attached to a rotating shaft turned by an outside energy source such as an electric motor or an internal combustion engine. Gears are used in many mechanical devices, including automotive transmissions, carpenter's hand drills, elevator lifting mechanisms, bicycles, and carnival rides such as Ferris wheels and merry-go-rounds.

 

Gears can be used in several different configurations. Two gears may be connected by directly touching each other, as in an automotive transmission. In this arrangement, one gear spins clockwise and the other rotates counterclockwise. Another possible configuration is to have two gears connected by a loop of chain, as on a bicycle. In this arrangement, the first gear rotates in one direction, causing the chain to move. Because the chain is directly connected to the second gear, the second gear will immediately begin to rotate in the same direction as the first gear.

 

Many times a system will use two gears of different sizes, as on a ten-speed bicycle. This will allow changes in speed of the bicycle or machine.

 

Problems about gears will always involve rotation or spinning. The easiest way to approach test questions that involve gears is to draw a diagram of what the question is describing, if one is not already provided. Use arrows next to each gear to indicate which direction (clockwise or counterclockwise) it is rotating.

 

Pulleys

 

A pulley consists of a wheel with a grooved rim in which a pulled rope or cable is run. Pulleys are commonly used with ropes or steel cable to change the direction of a pulling force or to add mechanical advantage.

 

Pulleys are often used to lift things. For instance, a pulley could be attached to the ceiling of a room. A rope could be run from the floor, up through the pulley, and back down to a box sitting on the floor. The pulley would allow you to pull down on the rope and cause the box to go up. That is, the pulley causes a change in direction of the pulling force.

 

Another common use for a pulley is to connect an electric motor to a mechanical device such as a pump. One pulley is placed on the shaft of the motor, and a second pulley is placed on the shaft of the pump. A belt is used to connect the two pulleys. When the motor is turned on, the first pulley rotates and causes the belt to rotate, which in turn causes the second pulley to rotate and turn the pump. This arrangement is very similar to the previous example of a bicycle chain and gears.

 

You may have seen pulleys used in a warehouse to lift heavy loads. Another use for a pulley is on a large construction crane. The cable extends from the object being lifted up to the top of the crane boom, across a pulley, and back down to the electric winch that is used to pull on the cable. In this situation, the pulley again causes a change in direction of the pulling force, from the downward force of the winch that pulls the cable to the upward movement of the object being lifted.

 

Levers

 

A lever is a very old mechanical device. A lever typically consists of a metal or wooden bar that pivots on a fixed point. The object of using a lever is to gain a mechanical advantage. Mechanical advantage results when you use a mechanical device to make a task easier; that is, you gain an advantage by using a mechanical device. A lever allows you to complete a task, typically lifting, that would be more difficult or impossible without the lever.

 

The most common example of a lever is a playground seesaw. A force (a person's weight) is applied to one side of the lever, which causes the weight on the other side (the other person) to be lifted. However, since the pivot point on a seesaw is in the center, each person must weigh the same or things do not work well. A seesaw is a lever with no mechanical advantage. If you push down on one side with a weight of 10 pounds, you can only lift a maximum of 10 pounds on the other side. This is no great advantage.

 

This brings us to the secret of the lever: To lift an object that is heavier than the force you want to apply to the other side of the lever, you must locate the pivot point closer to the object you want to lift. If two 50- pound children sit close to the center of the seesaw, one 50-pound child close to the end of the board on the other side will be able to lift them both.

 

Test questions about levers will typically require a bit of math (multiplication and division) to solve the problem. There is one simple concept that you must understand to solve lever problems: The product of the weight to be lifted times the distance from the weight to the pivot point must be equal to the product of the lifting force times the distance from the force to the pivot point. Stated as an equation: w × d1 = f × d2.

 

For example, Bill has a 15-foot long lever, and he wants to lift a 100-pound box. If he locates the pivot point 5 feet from the box, leaving 10 feet between the pivot point and the other end of the lever where he will apply the lifting force, how hard must he press on the lever to lift the box?

 

 

Use the lever formula, w × d1 = f × d2.The weight of 100 pounds times 5 feet must equal 10 feet times the force: 100 × 5 = 10 × force. Using multiplication and division to solve for the force, you get 50 pounds of force that Bill must apply to the lever to lift the box.

Fasteners

 

A mechanical fastener is any mechanical device or process used to connect two or more items together. Typical examples of fastening devices are bolts, screws, nails, and rivets. Processes can be used to mechanically join items together, including gluing and welding. There are also unique mechanical fasteners such as "hook and loop," which consist of two tapes of material with many small plastic hooks and loops that stick together. Hook and eye fastening tape—Velcro©—is the most common fastener used on firefighters' turnout gear, part of the complete personal protective equipment that protects firefighters from exposure to the products of combustion.

Springs

 

A spring is an elastic mechanical device, normally a coil of wire, that returns to its original shape after being compressed or extended. There are many types of springs, including the compression coil, spiral coil, flat spiral, extension coil, leaf spring, and torsional spring.

 

Springs are used for many applications such as car suspensions (compression coil and leaf springs), garage doors (extension coil and torsion springs), wind-up clocks (flat spiral and torsion springs), and some styles of ballpoint pens (compression coil).

 

In the kinds of questions you are likely to be asked on the firefighter exam, you can assume that springs behave linearly. That is, if an extension spring stretches one inch under a pull of ten pounds, then it will stretch two inches under a pull of 20 pounds. In real life, if you pull too hard on a spring, it will not return to its original shape. This is called exceeding the spring's elastic limit. Your exam is not likely to deal with this type of spring behavior.

 

If several springs are used for one application, they can be arranged in one of two ways—in series or in parallel. The easiest way to remember the difference is that if the springs are all hooked together, end to end, then you have a series of springs. The other option is for the springs not to be hooked together but to be lined up side by side, parallel to each other. If two springs are arranged in series, they will stretch much farther than the same two springs arranged in parallel under the same pulling force. This is because in series, the total pulling force passes through both springs. If the same springs are arranged in parallel, the pulling force is divided equally with half going through each spring.

 

 

The key to solving spring problems is to draw a diagram of the arrangement, if one isn't already provided, and follow the pulling force through the system.

Valves

 

A valve is a mechanical device that controls the flow of liquids, gases, or loose material through piping systems. There are many types of valves, including butterfly valves, gate valves, plug valves, ball valves, and check valves.

 

A valve is basically a gate that can be closed or opened to permit the fluid or gas to travel in a particular direction. The type of exam question you are likely to see that involves valves will be one in which you must follow a piping flow diagram through several sets of valves. These problems are best approached by taking your time and methodically following each branch of the piping system from start to finish.

Gauges

 

Gauges are used to monitor the various conditions and performance of mechanical machines such as pumps and internal combustion engines, as well as to monitor the surrounding atmospheric conditions, which could indirectly affect a particular machine.

 

Gauges are usually marked with the units they are measuring. A few examples of different types of units are:

degrees Celsius or Fahrenheit for temperature gauges

pounds per square inch (psi) for pressure gauges

meters (or sometimes feet) for elevation gauges

 

You must be very careful to recognize and understand the units of a gauge that appear in a test question. For instance, a temperature gauge (commonly called a thermometer) could use either degrees Fahrenheit or degrees Celsius. Mistakes on units can cause major problems, so be careful! The table on page 200 shows some common types of gauges, what they measure, and the kind of units they use.

 

Gauges are sometimes marked with warnings about limits of safe operation. Most gauges on fire apparatus are now color coded, and many newer pieces of apparatus are equipped with digital gauges with audible warnings. For instance, an oil pressure gauge on an internal combustion engine may show a maximum safe working pressure of 15 psi. If you are asked about the safe operation of a device with a gauge on it, you should pay careful attention to any markings that show such a limit.

 

 

Pumps

 

A pump is a device used to transfer a liquid or a gas from one location, through a piping system, to another location. For example, a fire engine is a large, self-propelled pump capable of delivering a large volume of water at varying pressures. There are many different types of pumps, including centrifugal pumps, positive displacement pumps, metering pumps, diaphragm pumps, and progressive cavity pumps.

 

Generally speaking, a working pump consists of the pump itself (case, bearings, impeller, seals, shaft, base, and other components) and an outside energy source. The outside energy source could be an electric motor, internal combustion engine, or battery to provide mechanical energy to the pump. This energy causes the inner workings of the pump to propel the liquid or gas through the piping system. The flow rate at which the liquid or gas is pushed through the piping system is typically measured by a flow meter in units of gallons per minute (gpm) or cubic feet per minute (cfm).

 

Pumps are used for many purposes. Additional examples include gasoline pumps used to pump the gasoline from a holding tank into your car, water pumps to transfer drinking water from a reservoir to your house or business, and industrial pumps used to move industrial fluids such as chemicals or waste products from one tank to another inside a plant. A car also uses pumps to pump fuel from the gas tank to the engine and to pump coolant from the radiator to the engine block.

 

Systems That Use Mechanical Devices

 

Many mechanical devices are actually a combination of several simple devices that work in conjunction to form a group of interacting mechanical and electrical components called a system. Some of the systems most likely to appear on the exam are discussed below.

 

Internal Combustion Engines

 

Internal combustion engines (ICEs) are commonly used to drive many mechanical devices. However, they are very complex mechanical devices themselves. ICEs are used in cars, trucks, construction equipment, and many other devices. They can be fueled by gasoline, diesel fuel, natural gas, or other combustible fossil fuels.

 

An ICE is a system composed of dozens of individual mechanical (as well as electrical) systems. A few of the major systems within an ICE are discussed below

 

The Cooling System

 

The purpose of the cooling system is to dissipate the heat generated by the engine. The system consists of a pump that moves the coolant from the radiator through piping to the engine block, where it becomes hot, and then back out to the radiator where the liquid coolant is cooled.

The Pistons, Tie Rods, and Crankshafts

 

The pistons, tie rods, and crankshafts are all parts of the inner workings of an ICE. In a gasoline-powered engine, a spark plug ignites the fuel and air mixture inside the cylinder, forcing the piston down. In a diesel engine, the ignition of the fuel is caused by the heat of compression of the air in the cylinder. At just the right time in the cycle, fuel is injected into the cylinder, causing an explosion that forces the piston down. In both types of engines, the piston is mechanically linked to a tie rod, which in turn is linked to a crankshaft. The up-and-down motion of the cylinder is changed into a rotational movement by the crankshaft. The crankshaft drives a transmission, which is a gear box. The transmission sends the power developed by the engine to the wheels of the vehicle, the workings of the pump, or whatever device the ICE is powering. Diesel power is the most common ICE installed in fire apparatus.

 

The Fuel Pump

 

Fuel, usually gasoline or diesel fuel, is transferred to the engine from the fuel tank (or tanks) by this pump, which is either a mechanically driven device or, as is now more common, electrically driven. The fuel pump delivers fuel to a carburetor (gasoline) or fuel injection system (diesel and newer gasoline engines), which distribute the fuel under pressure in a spray to the proper cylinder. Many devices that were formerly mechanically driven are now replaced by computer controlled devices.

 

The Throttle Governor

 

A throttle governor is a mechanical or electronic device that is used to control the speed of an ICE. In older motor vehicles, it is a spring device that works directly on the gas pedal. In more modern motor vehicles, it is an electronic device that limits the speed of the engine. A throttle governor can be used to limit or maintain the vehicle's speed and, on fire apparatus, to maintain speed at a set rate when the vehicle is used to power pumps, hydraulics, or operate auxiliary machinery when stationary.

Motor Vehicles

 

Motor vehicles are among the most complex assemblies of mechanical and electronic devices in existence. A piece of fire apparatus is among the most complex of all motor vehicles with hydraulic systems, power systems, pumps, compressed air, and lighting systems, to name a few. Today, computers have taken over more and more of the work that had previously been done by mechanical devices. All of these systems operate in addition to the normal subsystems discussed next.

 

The Brakes

 

Motor vehicle brakes can be of several types. Originally, brakes were mechanical, using direct pressure on a brake pedal and transferring that pressure by linkages to pads that applied the pressure to a drum or rotor attached to the axle. That friction slows or stops the vehicle. To increase the pressure from the braking system on the axles to control heavier vehicles or to reduce the strain on the driver, hydraulics have replaced mechanical linkages. Now when the brake pedal is depressed, a hydraulic cylinder forces fluid through brake lines that connect the main cylinder to hydraulic cylinders at each wheel. The fluid then forces the pads onto the rotors, which slows or stops the vehicle. To further increase the pressure on the pads, pumps can be added to increase the hydraulic pressure. With large vehicles, such as fire apparatus, even this type of braking may not provide the level of safe braking that is needed. Large vehicles use air brakes to slow or stop the vehicle. In these brakes, air is used either to keep the brake pads off the cylinder or to apply the pressure. These work in a similar way to the others described, but are designed to apply brakes any time air pressure is lost or suddenly reduced. Once again, brake systems are being computerized and now have features that prevent brakes from locking up and causing the vehicle to skid.

 

The Steering Assembly

 

The steering wheel is attached to the tip of the steering column. In older vehicles, the bottom of the column was directly attached to the wheels by a series of gears and levers, so that if the steering wheel was turned to the right, the vehicle turned right, and vice versa. Today, the steering system of a vehicle employs hydraulics either to assist in the movement of the wheels or to actually move them. Modern steering systems allow the vehicle operator to turn the vehicle with much greater ease than older steering systems.

 

The Exhaust System

 

As each cylinder fires, the combustion produces hot gases that expand in the confined space, forcing the piston down. For the engine to continue to function, it must exchange the burnt gases for fresh air and then, at the right moment, fuel. This exchange of combustion gases for fresh air is the job of the exhaust system. From an exhaust valve or from a series of ports, the burnt gases are drawn into a device called an exhaust manifold that gathers the gases from all the cylinders. This is connected to welded piping that passes the exhaust gases through a scrubbing device to remove harmful gases, changing them to harmless exhaust gases. This device, frequently called a catalytic converter, discharges the scrubbed exhaust gases through a muffler, which is an acoustical chamber that reduces the engine noise.

 

Bicycles

 

A bicycle is not nearly as complex as an automobile. However, it too uses several mechanical devices.

The chain drive. The pedals are connected to the drive gear. A chain is used to connect the drive gear to the gears on the rear wheel.

The frame. Many welded joints are used to hold the frame together.

The suspension system. Many newer bikes have suspension systems. The front wheel may use a hydraulic shock absorber. The rear wheel may use two springs in parallel to reduce shock to the rider.

 

Brushing Up on Related Topics

 

Some mechanical aptitude questions may require the use of math or science to determine the correct answer. This chapter cannot cover all the possible questions you might be asked on the firefighter exam, but here are suggestions for ways to increase your knowledge of math, science, and general mechanical aptitude.

 

Math

 

The required mathematical skills are primarily arithmetic (addition, subtraction, multiplication, and division) and geometry (angles and shapes). The arithmetic involved is almost always fairly simple. If you had trouble with arithmetic or geometry in your past schooling, you can brush up by reading the math chapter of this book. If you still want more help, pull out your old high school math book or check out a basic math book from the library.

 

Science

 

Science subjects such as physics, materials science, thermodynamics, and chemistry are confusing for some people, but they needn't be. Science is real, seen in everyday life. You see science in action dozens of times every day. A car is stopped by brakes, which use friction (physics). A magnet adheres to the refrigerator due to the properties of the magnet and carbon steel of which the door is made (materials science). A pot of water boils when you set it on the stove and turn on the burner (thermodynamics). A tomato plant grows through the chemical reaction of sunlight, water, and food (chemistry). This chapter has reviewed many of the scientific concepts that are involved in mechanical devices. Again, as with math, you may have science books from previous schooling that you can use to help you solidify your scientific knowledge. If not, the library is full of scientific resources.

 

General Mechanical Aptitude

 

Mechanical devices are such an integral part of everyday life that there are many real-life sources you can investigate to gain more knowledge of their design and use. A construction site is a great place to visit for a day to learn more about hand tools, cranes, pumps, and other devices. Ask the construction supervisor if you can take a tour.

 

Another alternative would be to visit an automotive repair shop. Internal combustion engines, lifts, levers, and hand tools are only a few of the types of mechanical devices you could see in use. Yet another possibility would be to visit a local manufacturer in your town. Examples include a foundry, a sheet metal fabricator, an automotive manufacturer, or a pump manufacturer. Look in the phone book under "manufacturing" for possibilities.

 

Sample Mechanical Aptitude Questions

 

1. Which of the following tools is used to smooth or level a piece of wood?

a wrench

a screwdriver

a plane

a hammer

 

 

2. A compass is used for what purpose?

to measure angles

to tighten and loosen nuts and bolts

to drive and remove nails

to draw circles of various sizes

 

3. Which of the following is NOT a hand tool?

a winch

a level

a compass

a chisel

 

 

4. Vice grips are a type of

ax.

wrench.

ladder.

mechanical jack.

How can gears be used to change the speed of a machine?

 

5. How can gears be used to change the speed of a machine?

use more gears

use two gears of the same size

use two gears of different sizes

use two large gears

 

6. What is the main function of a pulley?

 

to increase the strength of a construction crane

to override the power of an electric motor

to add energy to a system

to change the direction of a pulling force

 

7. Steve has a lever whose pivot point is 3 feet from the 50-pound box he      wants to lift. Steve is standing at the other end of the lever, 6 feet from the pivot point. How much force must he apply to lift the box?

 

50 pounds

25 pounds

100 pounds

6 pounds

 

8. Which of the following is NOT a mechanical process for fastening?

welding

buttoning

bolting

covalent bonding

 

9. When three identical springs are arranged in series and a pulling force of      10 pounds is applied, the total stretch is 9 inches. If these same three      springs were arranged in parallel and the same 10-pound force were      applied to the new arrangement, what would be the total distance of      stretch?

 

3 inches

4.5 inches

9 inches

18 inches

 

 

 

 

11. What type of gauge uses units of rpm?

a pressure gauge

a tachometer

a speedometer

a thermometer

 

12.What type of outside energy source could be used to operate a pump?

     a battery

an internal combustion engine

an electric motor

all of the above

 

13. What type of mechanical device is used to aid in cooling an internal       combustion engine?

a pump

a lever

a gauge

a hammer

 

14.Of the following mechanical devices on an automobile, which one uses      friction to accomplish its purpose?

the steering system

the exhaust system

the braking system

the internal combustion engine

 

15. The suspension system on a bicycle is likely to use which of the         following mechanical devices?

a chain

a pulley

a gear

a spring

 

16.The tops and caps of your department's fire hydrants have a hexagonal      stud extending about an inch from the base. Which of the following basic      tools could you use to open the hydrant or remove the cap?

pliers

wrench

screwdriver

lever

 

17. What gauge could be used to test the amount of water streaming from a        hydrant?

tachometer

pressure gauge

speedometer

flow meter

 

Answers

1.c. See the table under "Carpenter's Tools" earlier in this chapter for the functions of the items listed.

 

2.d. As defined under "Carpenter's Tools," a compass is used to draw circles.

 

3.a. A level, a compass, and a chisel are all carpenter's hand tools.

 

4.b. Vice grips are a kind of wrench.

 

5.c. Changing gears on a ten-speed bicycle is a good example of using different-sized gears to change speed.

 

6.D. Pulleys are used to change not the strength of a force but its direction.

 

7.b. Apply the distance formula, w × d1 = f × d2, to come up with the equation 50 × 3 = f × 6. Solve for the unknown f by multiplying 3 times 50 to get 150 and then dividing by 6 to get 25 pounds.

 

8.d. A covalent bond is a chemical bond. Welding, buttoning, and bolting are all mechanical fastening processes.

 

9.a. The total pulling force will be divided equally, with each spring experiencing one-third of the total force. Since the force is divided by 3, the amount of movement will be divided by 3 also. The original configuration stretched 9 inches, so the new arrangement will stretch only 3 inches.

 

10.b. A tachometer measures rotation in units of revolutions per minute or rpm.

 

11.d. Any of the energy sources listed could be used to operate a pump.

 

12.a. As discussed in the section "Internal Combustion Engines" earlier in this chapter, a pump is used to help cool an ICE.

 

13.c. The braking system uses friction to slow or stop the rotation of the wheels.

 

14.d. Springs are commonly used in suspension systems.

 

15.b. A wrench is used to turn a bolt-like head. Although pliers could be used, they would tend to slip. Both a lever and a screwdriver would be useless in this instance.

 

16.d. Flow meters measure the volume of flow within a piping system or flowing from a piping system. A pressure gauge would show you the pressure of the water, but you are not interested in that. You need to measure flow, the volume (amount of water), not the pressure (force of water). Tachometers and speedometers measure machinery speed, not water speed or volume.

 

How to Answer Mechanical Aptitude Questions

 

Read each problem carefully. Questions may contain words such as not, all, or mostly, which can be tricky unless you pay attention.

 

Read the entire question once or even a few times before trying to pick an answer. Decide exactly what the question is asking. Take notes and draw pictures on scratch paper. That way you won't waste time by going in the wrong direction.

Some questions will require the use of math (typically addition, subtraction, multiplication, and division) and science. In these situations, think about what you have learned previously in school.

Use your common sense. Some mechanical devices can seem intimidating at first but are really a combination of a few simple items. Try to break complicated questions down into smaller, manageable pieces.

 

Answer the questions that are easiest for you first. You do not have to go in order from start to finish. Read each question and, if you are not sure what to do, move on to the next question. You can go back to harder questions if you have time at the end.

 

Many mechanical devices are commonly used in everyday life. You do not have to be a mechanic or an engineer to use these devices. If something seems unfamiliar, try to think of items around your house that might be similar.

Don't be intimidated by unfamiliar terms. In most instances, there are clues in the question that will point you toward the correct answer, and some of the answers can be ruled out by common sense.

 

Physical Science basics:

 

Archimedes (ca. 287–212 BC), the great Sicilian inventor, physicist, mathematician, and engineer, is credited with saying, "Give me a place to stand and I will move the earth." He understood the concept of using simple machines to gain a mechanical advantage to move or lift heavy objects with less force.

 

The first simple machines were probably wooden levers used to move large boulders and sharp rocks used as wedges to scrape off the skins of dead animals. Around 3,000 BC, logs were used as rollers to move heavy objects. This concept developed into the wheel and axle simple machine. Some 200 years later, the builders of Stonehenge used levers, rollers, and pulleys. Soon afterward, inclined planes, ramps, and rollers were used to build the Great Pyramids of Egypt. Progress continued with the development of the screw principle by Archimedes in the third century BC. Early civilizations throughout the world continued to develop and invent simple machines to construct their buildings and facilitate work. Progress was steady throughout the next 20 centuries—with the invention of the spinning wheel, compass, printing press, telescope, mechanical clock, and engine—but it was given a big leap forward by improvements to the steam engine by James Watt of England in the mid-eighteenth century. That ushered in the Industrial Revolution and the machine age for modern man.

Physical Science Definitions and Formulas

 

Force is generally thought of as a push, pull, dropping, stretching, or squeezing of an object that results in a change in the shape and/or a change in the motion of the object. Examples of forces include gravity, magnetism, and electricity.

 

Force is measured in the SI (System International), or metric system, in units called Newtons (N), for the great English mathematician and physicist Isaac Newton (1642–1727). In the British system of measurement, force is measured in pounds × foot.

 

Force (F) = Mass (M) × Acceleration (A)

 

Mass is the quantity of matter of an object. In the metric system, the unit of measurement of mass is the kilogram (kg).

 

Weight is a force originating when a mass is acted on by gravity. Weight (W) is the product of an object's mass (m) and the acceleration of gravity (g) at the location of the object, or W = mg. The units of weight in the SI are Newtons. Weight is also measured by the gram in the metric system and by the ounce or pound in the British system.

 

Velocity is a measure of how fast an object is moving in a given direction. In the metric system, velocity is measured in meters per second (m/s). In the British system, the measurement is in miles per hour (mph).

 

Momentum is the term used when mass has a velocity. Its unit of measure has no metric or British name.

 

Momentum = Mass × Meters/Second

 

Acceleration is the rate of change of velocity. The metric unit of measure for acceleration is meters per second per second (m/s/s). The British system measures acceleration in feet per second per second (ft/s/s).

 

Length is measured in meters (m) in the metric system. In the British system, length is measured in feet (ft).

 

Newton is a measure of the amount of force required to accelerate a mass of one kilogram (kg) at a rate of one meter (m) per second (s) per second (s).

 

1 N = 1 kg × m/s/s

 

Friction is a force that reduces the motion of objects. It occurs when two objects rub against each other with heat as a byproduct. Friction is reduced by polishing a surface and by using lubricants (oils and greases) and rollers to allow objects to move more easily.

 

Work is the use of force to cause motion. It is the transfer of energy through motion. Energy (work) can never be destroyed; it can only be transferred. For work to take place, a force must be exerted through a distance. The amount of work performed depends on the amount of force that is exerted and the distance over which the force is applied.

 

Work (W) = Force (F) × Distance (D)

 

Mechanical work is measured by the joule (J), named after the nineteenth century English physicist James Prescott Joule. (One joule is equal to 1 Newton multiplied by 1 meter.)

 

1 J = 1 N ×1 m

 

The British system measures mechanical work in foot-pounds (ft-lb).

 

Torque is a twisting force that occurs when the force is not applied to the object's center of mass.

 

Power is the rate at which work is performed. It is derived by measuring work per unit of time. The metric system unit used to measure power is the Watt (W). One watt is equal to 1 joule per second.

 

1 W = 1 J/s

 

1000 Watts = 1 Kilowatt (KW)

The British unit of measurement for power is the horsepower (HP).

 

1 HP = 550 ft-lb/s = 746 watts

 

 

 

 

Some