ENERGY AND FORCE 



41 



is exhausted from a tube arranged for the purpose, it 

 will be found that the feather will fall as rapidly as the 

 stone, because it is not influenced by the resist- 

 ance of the air (Figure 33). 



Energy. It is quite easy for us to realize 

 that energy may be transferred from one body to 

 another. A moving billiard ball strikes another 

 ball " full," and apparently all of its energy is 

 immediately transferred to the second ball, while 

 the first ball stops. In fact, however, not quite all 

 the energy of the first ball was transferred to the 

 second. Part of it went into other forms of energy, 

 as heat and sound. The billiard ball in motion 

 possesses energy of motion. The name which 

 scientists have given this form of energy is kinetic 

 energy, a name derived from the Greek 

 word kineo, moving. In recent years 

 we have had a new word from this FlG - 33 - 

 root, kinematograph or cinematograph. Ki- 

 netic energy is one of the several distinct 

 forms of energy which we shall learn about 

 in our work in science. 



We may pull a stone away from the earth, 

 and the earth pulls it back again. If,- however, 

 we pull the stone away from the earth and 

 support it there, the stone will have the power 

 of doing work when it is freed. This form 

 of energy is called potential energy. There 

 are numerous instances of such energy. The 

 weights of a clock when lifted possess potential 

 energy (Figure 34). Confined steam furnishes 

 another example. It is the energy of strain or deforma- 

 tion as well as the energy due to position. Potential 



FIG. 34. 



