CHAPTEE II 

 FORCE AND THE LAWS OF MOTION 



NEWTON'S LAWS 



17. The laws of motion, as we have said, form the material sup- 

 plied by experimental mechanics for theoretical mechanics to work 

 with. These laws have been stated in compact form by Newton : 



LAW I. Every body continues in its state of rest, or of uniform 

 motion in a straight line, except in so far as it is compelled by 

 impressed force to change that state. 



LAW II. The rate of change of momentum is proportional to the 

 impressed force, and takes place in the direction of the straight line 

 in which the force acts. 



LAW III. To every action there corresponds an equal and oppo- 

 site reaction. 



18. These laws introduce several new terms, "force," "mo- 

 mentum," " action," " reaction," which must be explained before 

 the laws can be fully understood. 



The first law involves the idea of motion, which has already 

 been discussed, and of force, which is new. 



The .word " force " is in common use. It is associated in the 

 first instance with muscular effort ; for example, we exert force in 

 pushing against an obstacle. Scientifically, however, the word has 

 a wider use ; we say, for instance, that two railway trucks when 

 they collide exert force on one another, and that the earth exerts 

 force on all bodies, causing them to fall towards it unless they are 

 supported in such a way that they resist this force. 



The first law of motion, in fact, explains what is to be understood 

 by force : it is that which tends to change the state of rest of a 

 body, or of uniform motion in a straight line. 



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