140 THE REALITIES OF MODERN SCIENCE 



or later, because of the "frictional forces." If the 

 friction is reduced the body maintains a state of 

 motion for a longer time. We, therefore, reason that 

 if all opposing forces could be removed the body would 

 continue to move with uniform velocity. 



This law was stated about two centuries before the 

 reality of energy had been recognized. Changes in 

 energy are the concomitants of changes in motion and 

 to-day many scientists consider the former to be the 

 causes of the latter. We have seen that energy may 

 be either potential or kinetic and also that decreases 

 in potential energy result in increases in kinetic energy 

 or vice versa. 



A body in motion or one at rest will have the same 

 kinetic energy indefinitely unless it acquires more at 

 the expense of some source of energy or unless it im- 

 parts its energy to other bodies or to parts of itself. 

 The latter happens when there is friction, for the body 

 then transfers some of the kinetic energy, which it 

 has as a bulk, to its own and adjacent molecules. 



A moving body is retarded if its motion is in such a 

 direction as to increase the potential of the system which 

 it forms with another body. As a body which is 

 thrown into the air moves away from the earth the 

 potential energy of the system increases. The kinetic 

 energy of the body is thus converted into potential 

 energy, and at the instant when the conversion is just 

 completed the body is at rest. 1 It then falls and the 



1 Of course, if the kinetic energy is greater than the greatest 

 amount of potential energy which the body can add to the system 

 by moving to any distance whatever, then it cannot thus come to 

 rest. This is the case for comets. 



