148 THE REALITIES OF MODERN SCIENCE 



Newton recognized that the change in the momen- 

 tum of a body, which takes place when it is acted upon 

 by a force, is merely one aspect of the phenomenon. 

 He reached a conclusion, partly by experiment and 

 partly by inference, that in all such cases an equal 

 and opposite change in momentum must occur in some 

 other body. 1 As he stated it in his third law of motion: 

 " Action and reaction are equal and opposite; or in 

 other words, the mutual actions of two bodies are 

 always equal and oppositely directed." We recog- 

 nize this fact in such cases as that of a man jumping 

 from a. small boat. As the man jumps forward, he 

 kicks the boat backward. The man and the boat are 

 put into motion in opposite directions. According to 

 Newton, the momentum of the man, that is, the product 

 of his mass and his velocity, is equal to the momentum 

 of the boat. If, then, the boat has a large mass it will 

 acquire only a small velocity. If its mass is very large 



J It is therefore usual to say that there is a "conservation" of 

 momentum. The use of the term is unfortunate, particularly in 

 elementary textbooks, for it implies that momentum is an indestruct- 

 ible in the same sense as is energy. To readers who remember 

 the emphasis in such texts it may have appeared that momentum 

 is a reality like energy. The principle of the conservation of mo- 

 mentum is a recognition of the characteristic inertia of matter which 

 is implied in Newton's first law of motion. The parts of a system 

 cannot of themselves alter the location of their center of mass. For 

 example, at any instant after the explosion of a shell the configura- 

 tion of the fragments will be such that their center of mass occupies 

 the position in space which the center of mass of the shell would have 

 occupied at that instant if the explosion had not occurred. The 

 principle is equivalent to that of the "conservation of the center 

 of mass." Both are implicit consequences of inertia, the distinctive 

 quality of bodies which may acquire kinetic energy. Momentum 

 is not, therefore, to be considered a reality in the sense in which 

 the word has been applied to matter and to energy. 



