FORCE, A SPACE RATE OF ENERGY 151 



recoil to the man as the ball leaves his fingers? Ob- 

 viously because he serves only as a medium which trans- 

 mits the reaction of the ball to the earth. 



The action of the man is upon the ball and its motion 

 depends only upon its mass and the force which he 

 exerts. Similarly the reaction of the ball on the man 

 is not a matter which has a further concern with the 

 motion of the ball. If there were no stress between the 

 man and the earth his momentum would be equal 

 and opposite to that of the ball. Whether he takes 

 up the momentum himself or merely transmits it to 

 the earth does not alter the equality of the action 

 and the reaction. 



Now consider the case of A pushing upon B, who in 

 turn braces against the ground. As long as B's body 

 will stand the stress, to which it is subjected between A 

 and the earth, there is no motion. When it will no 

 longer, or when his feet slip, then he moves. So far 

 as his motion is concerned he is to all intents and pur- 

 poses a bowling ball. His reaction on A makes no 

 difference whatever hi his own motion, although it 

 will determine the motion of A unless the latter suc- 

 ceeds in imparting it to the earth. That this is so, 

 is readily to be grasped in case one pictures A and B as 

 engaged in their struggle on a polished frictionless 

 floor. Then if A grasps B he may thrust the latter 

 away from himself. As he does so, however, there is 

 a reaction of B on him. The result is that both A and 

 B move with equal momenta but in opposite direc- 

 tions exactly as in the case of the man and the boat. 

 If the floor does not have a negligible friction the heavier 

 man, of course, has the advantage, for he will transmit 



