184 



ELEMENTS OF GENERAL SCIENCE 



engine demonstrates that it has the ability to do work. This 

 ability, or capacity, to perform work is called energy. 



The energy possessed by a weight in a condition similar 

 to that in our last example is utilized in clocks which are 

 run by weights (fig. 92). In this case, when the clock is 

 wound, the cord is wrapped on a small windlass and the 

 weight is lifted to the top of the clock. In 

 this position it possesses energy enough to 

 turn the wheels of the clock for some time. 

 When the weight has reached the bottom 

 of its 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 possesses energy, 

 which may be utilized in driving a nail. 



199. Kinfls of energy. The weight in the 

 clock possesses energy because of its posi- 

 tion. When it is wound up and is sus- 

 pended 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, the weight is not in a position 

 to do further work. The energy which it possessed was 

 dependent upon its position. 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 



FIG. <J2. A clock 

 run by weights 



