ENERGY 111 



will acquire a greater speed and hence take less time to 

 fall than if part of the energy must be expended in 

 accelerating the molecules of air which are adjacent 

 to its path. 



The greater the amount of energy which friction 

 subtracts from the system, the smaller the kinetic 

 energy imparted to the body and hence the more slowly 

 does it move. Now, it is possible to arrange the fric- 

 tion which a falling body must overcome so as to sub- 

 tract all of its energy. When this is done, the body, 

 of course, descends so slowly and uniformly that it is 

 scarcely moving when it reaches its final resting place. 

 We accomplish this every day without realizing the 

 physics of it. When one sets a glass of water on a table 

 he does so in such a way that it doesn't bump ; 

 that is, he makes sure that it has no kinetic energy, 

 retarding its fall by absorbing into his own muscles 

 the energy which is released as the glass descends from 

 a point of higher to one of lower potential. 



Perhaps the most striking illustration, however, is 

 the parachute which was used in the War for escaping 

 from observation balloons. This is merely a huge 

 silk umbrella without ribs. In the top is a small hole 

 through which the air can escape slowly and with 

 considerable friction. When the parachute is released 

 from the balloon it falls very rapidly, until the air 

 has opened it. Thereafter its descent is quite gradual. 

 When the parachute is working properly the bal- 

 loonist arrives at the earth without an unfortunate 

 amount of kinetic energy, for the energy which he 

 possessed at the higher altitude has been entirely con- 

 verted into molecular kinetic energy of the air. 



