Energy and Heat 123 



All moving bodies possess kinetic energy to a greater 

 or less extent, but a few examples of such bodies as 

 are capable of doing what is usually termed " work " 

 (by which we understand in ordinary affairs work 

 useful to man) are subjoined : 



(a) The water of a river. 



(6) A stone just before it leaves a sling. 



(c) A smith's hammer as it nears the anvil. 



(d) The hot air arising from a gas-flame. 



(e) A circular saw, when in use. 



79. A body may possess both kinds of energy at 

 one and the same time. The water at the top of a 

 waterfall is moving at a certain speed and hence pos- 

 sesses kinetic energy. By virtue of its elevated position 

 at the top of the fall, it is capable of doing work during 

 its fall to the bottom of the fall, i.e. it has a store of 

 potential energy in addition to its kinetic energy. At 

 the bottom of the fall, the water has lost the potential 

 energy due to its position, but is moving much faster 

 than it was before falling. That is to say, it has gained 

 kinetic energy during its fall, and we find that, if 

 measurements of both the loss of potential energy 

 and the gain of kinetic energy be made, they are 

 equal in magnitude. The potential energy is not lost, 

 but merely transformed into kinetic energy. This 

 relation between the loss of one kind of energy and 

 the simultaneous production of an equal quantity of 

 another form of energy is of universal application. 

 If we trace with great care a certain definite quantity 

 of energy of any kind through a series of manifestations, 

 during which it suffers any number of changes, we 

 are able to show that, although none of the energy 

 may be existing in its original form, exactly equivalent 



