80 ENERGY. 



motion but was at rest, it had the power of doing work by 

 reason of its elevated position with reference to the earth. 

 When the water of the running stream was at rest in the 

 lake among the hills it had a power of doing work, an 

 energy, which was not possessed by the waters of the 

 pond in the valley below. This energy or power results 

 from its peculiar position. Energy of motion is called 

 kinetic energy; energy of position is called potential 

 energy. 



159. Convertibility of Kinetic and Poten- 

 tial Energies. We may at any moment convert kinetic 

 energy into potential, or potential energy into kinetic. 

 One is as real as the other, and when it exists at all, exists 

 at the expense of a definite amount of the other. Imagine 

 a ball thrown upward with a velocity of 64.32 feet. As it 

 begins to rise it has a certain amount of kinetic energy. 

 At the end of one second it has a velocity of only 32.16 ft* 

 Consequently its kinetic energy has diminished. But 

 it has risen 48.24 ft, and has already a considerable poten- 

 tial energy. All of this potential energy results from the 

 kinetic energy which has disappeared. At the end of 

 another second, the ball has no velocity; it has reached the 

 turning-point and is at rest. Consequently, it has no 

 kinetic energy. But the energy with which it began its 

 flight has not been annihilated; it has been stored up in 

 the ball at a height of 64.32 ft. as potential energy. If at 

 this instant the ball be caught, all of the energy may be 

 kept in store as potential energy. If now the ball be 

 dropped, it begins to lose its potential and to gain kinetic 

 energy. When it reaches the ground at the end of two 

 seconds it has no potential energy, but just as much of the 



