150 THE REALITIES OF MODERN SCIENCE 



view 7i as the action and AI as the man's reaction. 

 Which one of a pair we consider the action is merely 

 a matter of point of view. 



When we consider the forces Ri and R 2 of the figure 

 we notice that the man is subjected to equal and op- 

 posite forces. This pair of equal and opposite forces 

 also constitute a stress. Similarly the forces AI and 

 A 2 constitute a stress. The man is exerting a pair of 

 equal and opposite forces, in other~words, a stress. 

 From the other point of view he is subjected to a stress. 

 In all cases of the mutual actions of bodies stresses 

 occur. 



For bodies at rest this idea is very evident when de- 

 scribed in terms of force with the loose meaning which 

 we have discussed above. Thus a body at rest on a 

 table acts downward on the table and is supported 

 by the upward reaction. When, however, a body in 

 motion is considered, the idea meets an instinctive 

 objection on one's part. Thus one says, "But if A 

 pushes B away, must not A push harder than B ? If 

 the reaction is just equal to the action, why doesn't 

 it just balance it and prevent any motion?" 



Consider first the case of a man acting on a bowling 

 ball instead of upon another man. If the ball is of 

 small mass it does not require as great an action upon 

 the man's part to give to it a definite acceleration. In 

 fact, neglecting air friction, the force which he is con- 

 scious of exerting depends upon the mass and accelera- 

 tion. These determine the resistance which his arm 

 meets in swinging the ball. If it was a gun which was 

 shooting the ball, the reaction of the ball on the gun 

 would be evidenced by the recoil. Why is there no 



