CHAPTER XVII 



MECHANICAL ENERGY AND HEAT 



201. Capacity for doing work. In all the experiments with 

 the system of pulleys which you may have performed in con- 

 nection with the preceding chapter, you yourself applied the 

 force, since you pulled at the end of the 

 cord. You might have obtained the same 

 results in another way. You might have 

 attached a weight of some sort to the end 

 of the cord, and if this were of the proper 

 size, it would, when released, descend and 

 cause the load carried by the movable pulley 

 to rise. It requires work to raise the load, 

 and it therefore follows that the weight upon 

 the end of the cord is able to do work. 

 Also the pull upon the cord might have 

 been supplied by an electric motor or by a 

 steam engine. In either case the weight, the 

 motor, or the engine demonstrates that it 

 has the ability to do work. This ability, or 

 capacity, to perform work is called energy. 

 202. Examples of bodies possessing energy. 

 The energy possessed by a weight in a con- 

 dition similar to that in our last example is 

 utilized in clocks which are run by weights 

 (fig. 90). 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 



180 



FIG. 90. A clock 

 run by weights 



