MECHANICAL ENERGY AND HEAT 181 



descent, the clock must be wound again. Most modern clocks 

 are driven by coiled springs, and in this case it is the tightly 

 coiled spring that possesses energy. A sled coasting downhill 

 possesses energy, as may be observed if it strikes any other 

 object on the way ; a cannon ball possesses energy, which is 

 expended upon objects in its path ; a moving hammer pos- 

 sesses energy, which may be utilized in driving a nail. 



203. Kinds of energy. The weight in the clock possesses 

 energy because of its position. When it is wound up and is 

 suspended upon the cord, it is able to descend, and in doing 

 so it will do work in turning the wheels of the clock. When 

 the cord is unwound, it is not in a position to do further work. 

 The energy which it possessed was dependent upon its posi- 

 tion. The same is true of the bent spring, which possesses 

 energy by virtue of the position of the parts of the spring 

 in relation to each other. This energy of position is called 

 potential energy. 



On the other hand, the cannon ball does not owe its energy 

 to position at all, but to the fact that it is moving rapidly. It 

 is the moving cannon ball and not the stationary one that is 

 effective for purposes of destruction. The hammer and the sled 

 also owe their energy to motion. Energy of motion is called 

 kinetic energy. 



204. Measurement of energy. Since energy is simply the 

 capacity to do work, it can be measured in the same units as 

 those which are used to measure work. Energy is measured 

 in foot-pounds. 



A 5-pound weight at a height of 2 feet possesses 5 times 2 

 foot-pounds of energy, since in order to place it there a force 

 of 5 pounds had to be applied through a distance of 2 feet. Thus 

 the energy which a body possesses is in general equal to the 

 work which has been done upon it. 



205. Work against friction produces heat. It is a common 

 experience that when work is done against friction, heat is 

 produced. Anyone who has attempted to break a wire by 



