MECHANICS 55 



motion the quantity of motion in each will then be 

 proportional to its velocity. 



If two unequal bodies are moving with different 

 velocities, their quantity of motion is jointly proportional 

 to their respective masses and velocities. 



But motions may not be of uniform velocity during the 

 time they last ; they may be continually accelerated or 

 retarded, so that their velocity varies from moment 

 to moment owing to some accelerating or retarding 

 cause. 



Matter itself must be regarded as absolutely inert 

 not inert, however, in the sense that it is more inclined 

 to rest than to motion, or that motion naturally tends 

 to come to an end. By calling motion "inert," it is 

 simply meant that matter is totally indifferent to either 

 rest or motion, and therefore it has been purposed 

 to speak of this quality as persistence rather than 

 inertia. 



The following are Newton's three laws of motion : 



(i) Every body continues in its state of rest, or of 

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

 by impressed foi'ces to change that state. 



Now there is no such thing as absolute rest, since, 

 as we have seen, every body tends to move in the direc- 

 tion in which gravity draws it, and only does not so move 

 because some other force prevents it. Therefore what 

 the first part of this law really asserts is, that when a 

 body is maintained in a certain state and position by 

 the combined action of two or more forces, such state 

 will continue till some other force changes the con- 

 ditions. 



The second part of the law affirms that a body in 

 motion tends to move uniformly in a straight line. This 

 necessarily means that its movement must continue in 



