ENERGY 103 



which is communicated to it, it is said to possess kinetic 

 energy. Of this a moving bullet, the falling weight of a 

 pile driver, and the rotating flywheel of an automobile 

 engine are examples. In the same way moving mole- 

 cules or the moving electrons of their atoms possess 

 kinetic energy. 



Kinetic energy is the only kind of which we may be 

 conscious, for there is no motion in the case of poten- 

 tial energy. In the wood beside the fireplace and the 

 oxygen in the room there exists a possible source of 

 energy, the potential energy of separation of two chem- 

 ical compounds, which is only manifested when they 

 are allowed to unite. In the raised weight of a pile 

 driver, in the coiled spring of a watch, in the powder 

 of a cartridge, in the water of lakes and reservoirs 

 which are above the surrounding ground, there is 

 potential energy. 



The weight of a pile driver possesses potential energy 

 because it may fall toward the earth, and hence it is 

 not the weight itself, but rather the system, of the weight 

 and the earth which attracts it, which possesses this 

 energy. In the same way the water of an elevated 

 reservoir does not itself possess the potential energy, 

 but rather the system of which it is a part. Potential 

 energy is due to the position or separation of the parts 

 of a system. In the case of the explosive mixture of 

 the cartridge this is also true, and the energy is re- 

 leased by allowing the combination of the separated 

 parts to take place. 



But where is the separation in the case of the coiled 

 watch spring? If anything, the parts appear to be 

 closer together when the spring is wound than when it 



