48 



MOTION AND FORCE 



it downward as far as c. The ball really moves along the line 

 ad to d; but as d is as far to the right of a as is 6, and as far 



below as is c, each force has had 



its effect. 



The second part of the 

 law means simply that the 

 greater the amount of force 

 used or the longer the time 

 that a force acts, the greater 

 will be the amount of mo- 

 tion caused. 





FIG. 30 



58. The Third Law. 



This law means simply that 

 whenever any body exerts 

 force upon another, the 

 second body in resisting 

 that action exerts the same amount of force upon the 

 first. If we strike a piece of wood with a hammer, 

 the hammer is stopped by the wood ; clearly, the wood 

 exerts force upon the hammer in stopping it. This 

 force is just equal to that exerted by the hammer, 

 for had it been less, the motion would not have been 

 stopped entirely; or had it been more, the hammer 

 would have been driven back. Force exerted in this 

 way, being caused by the action of another force, is 

 called reaction. Examples of reaction are common: a 

 boat exerts force upon the water as it moves, and the 

 water reacts upon the boat, tending to stop it; in the 

 same way the air reacts upon a train or any moving 

 object; it is well known that a bicycle rider can go 

 faster if he follows a moving shield. In all these cases 



