HOW WORK IS DONE 



materials, under given conditions, it varies with the 

 pressure, in a definite and measurable rate, which is 

 spoken of as the coefficient of friction for the particular 

 substances. It is very much greater where the two 

 substances slide over one another than where the one 

 rolls upon the other, as in the case of the wheel. The 

 latter illustrates what is called rolling friction, and in 

 practical mechanics it is used constantly to decrease 

 the loss as, for example, in the wheels of wagons and 

 cars. The use of lubricants to decrease friction is 

 equally familiar. Without them, as everyone knows, it 

 would be impossible to run any wheel continuously 

 upon an axle at high speed for more than a very brief 

 period, owing to the great heat developed through 

 friction. Friction is indeed a perpetual antagonist of 

 the mechanician, and we shall see endless illustrations 

 of the methods he employs to minimize its influence. 

 On the other hand, we must recall that were it rendered 

 absolutely nil, his machinery would all be useless. 

 The car wheel, for example, would revolve indefinitely 

 without stirring the train, were there absolutely no 

 friction between it and the rail. 



AVAILABLE SOURCES OF ENERGY 



We have pointed out that every body whatever con- 

 tains a certain store of energy, but it has equally been 

 called to our attention that, in the main, these stores 

 of energy are not available for practical use. There 

 are, however, various great natural repositories of 

 energy upon which man is able to draw. The 



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