ENERGY. 



81 



kinetic type as was given to it when it began to rise. This 

 illustrates in a simple way the important principle, the 

 transformation or convertibility of energy without 

 any change in its quantity. 



160. Energy a Constant Quantity. In the 



case of the ball thrown upward, at the start, at the finish, 

 or at any intermediate point of either its ascent or descent, 

 the sum of the two types of energy is the same. It may 

 be all kinetic, all potential, or partly both. In any case, 

 the sum of the two continually varying energies is 

 constant. Just as a man may have a hundred gold dol- 

 lars, now in his hand, now in his pocket, now part in his 

 hand and the rest in his pocket ; changing a dollar at a 

 time from hand to pocket or vice versa, the amount of 

 money in his possession remains constant, viz., one hun- 

 dred dollars. 



161. Pendulum Illustration. The pendulum 

 affords a good and simple illustration of kinetic and poten- 

 tial energy, their equivalence 



and convertibility. When the 

 pendulum hangs at rest in a 

 vertical position, as P#, it has 

 no energy at all. Considered as 

 a mass of matter, separated from 

 the earth, it certainly has po- 

 tential energy ; but considered 

 as a pendulum,it has no energy. 

 If the pendulum be drawn 

 aside to J, we raise it through 

 the space ah ; that is, we do 

 work, or spend kinetic energy upon it. The energy thus 



