Fig 291. One of the largest locomotives ever built (Courtesy of the Erie Railroad Company). This locomotive weighs 336 tons and can exert a pulling force of 60 tons. It can easily pull a train of 200 cars, each weighing 100 tons. It owes its pulling power to the friction of its drive wheels upon the rails.

Identifier: scienceforbeginn00barb (find matches)
Title: Science for beginners
Year: 1921 (1920s)
Authors: Barber, Frederic Delos, 1868- Fuller, Merton Leonard, 1872- joint author Pricer, John Lossen, 1871-1920, joint author Adams, Howard William, 1875- joint author
Subjects: Science
Publisher: New York, H. Holt and Company
Contributing Library: The Library of Congress
Digitizing Sponsor: The Library of Congress

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iction of great service to us.When we wish to haul a heavy load up a hill we make everyeffort to reduce friction. When we start down hill, however,we set the brakes, or possibly chain one rear wheel, in orderto increase friction. A locomotive owes its power to pull atrain to the friction between its drive wheels and the rails(Fig. 291). If this locomotive were attached to a train re-quiring a force of more than 60 tons to move it, the locomo-tive would be unable to start the train because its drive wheelswould then slip on the rails. A bicycle or an automobile islikewise propelled by the friction between its wheels and theground. Men working upon the ice wear ice creepers on theirshoes to increase friction between their shoes and the ice.After an ice or sleet storm, horses are almost useless for haul-ing loads unless they are sharp shod. Why? Give as manycases as you can where friction is of service to us. How dothe brakes of a railroad train bring the train to rest ? How MACHINES 483
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484 MACHINES, WORK, AND ENERGY is friction involved in feeding the paper through a type-writer or printing press ? Explain how a leather belt is usedto drive a sewing machine or a threshing machine. V. ENERGY AND ITS RELATION TO THE USE OF MACHINES 569. What is Meant by Energy?—Anything which is capa-ble of doing work possesses energy. Energy is the capacityfor doing work (Art. 81). Man, by putting forth effort,does work; therefore he possesses energy. A horse may dowork; therefore, he possesses energy. A steam engine, aslong as it is supplied with fuel and water and is properlycontrolled, can do work; therefore it possesses energy. Themachine by itself, i.e., without fuel and water, can do no work.It is the fuel which gives it the ability to do work. Theenergy comes from the fuel, not from the mechanism of theengine. We have seen in Chap. VII that it is from food thatman or the horse gets his supply of energy. 570. Motors.—Any machine or animal used to transformenergy into work i

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