110 THE REALITIES OF MODERN SCIENCE 



systems, as for example, to the molecules of the air 

 through which the body falls, then its kinetic energy is 

 1 ft. Ib. for each pound of the body. When it has fallen 

 to a point 18 feet above the zero of potential, it has 2 

 ft. Ibs. of kinetic energy for each pound of the body. 

 The decrease in potential energy is always equal to the 

 increase in kinetic energy, provided that none of the 

 available energy is allowed to get away from the system. 

 There is a change in the kind of energy going on steadily 

 as the body falls, but no change in the total amount 

 of energy possessed by the system. Such a system, 

 where energy is neither added nor subtracted by 

 some outside system, is called "conservative." 



In practice, however, we do not find conservative 

 systems. All the systems with which we have to do 

 either lose or gain energy from other systems. In the 

 case of a body falling through air some of the energy 

 is imparted to the molecules of the air which the 

 body pushes aside in falling. These air molecules are 

 set into motion ; that is, they are given kinetic energy. 

 The energy given to them is subtracted from the sys- 

 tem which we are considering. The system of earth 

 and body is not, then, a conservative system. If, 

 however, we consider the system to be composed not 

 only of the earth and the body but also of the molecules 

 of the air we do have a conservative system. 



In the story of Galileo's experiment we saw that a 

 falling body does not fall as quickly if there is fric- 

 tion. We now see from our knowledge of energy why 

 this is so. In falling through any given distance a 

 certain definite amount of energy is released. If all 

 this energy is available for accelerating the body, it 



