v MATTER IN RELATION TO MOTION 01 



to be wrong expressions of the order of nature, and evidently 

 there could be no sort of compulsion about what was wrong, and 

 was seen afterwards to be contrary to the general rule. 



This law, which Newton first stated as being always followed 

 by bodies in nature, means, first, that if a body is at rest, it will 

 remain still until there is some reason for its moving until some 

 outside influence, which is called a force, acts upon it. In fact the 

 law really supplies us with a definition of force. Nobody finds 

 any difficulty in understanding the rule so far. But when we 

 come to consider the second part of the law there is more 

 difficulty in grasping it every body will continue in a state 

 of uniform motion in a straight line, &c. An example is afforded 

 by a ball in moving uniformly along ice. We know that after 

 a time the ball comes to rest and therefore that it does not 

 continue in a state of uniform motion. But we know that it 

 moves for a longer time on ice than it would do on a road. The 

 ice is smoother than the road, and there seems to be a connec- 

 tion between the roughness or smoothness and the length of 

 time during which the ball moves. If we imagine smoother and 

 smoother ice, the ball will move for a longer and longer time, 

 and we conclude that if both the ball and the ice were perfectly 

 smooth, there is no reason why the ball should ever stop. The 

 roughness or friction is then, in our example, the "impressed 

 force" which causes the ball to change its state of uniform motion 

 for one of rest. If we could have a body in a state of uniform 

 motion outside the influence of what Newton has called " im- 

 pressed forces " it would afford us an example of perpetual 

 motion. But because we cannot eliminate these impressed forces 

 we cannot have perpetual motion. 



The first law of motion implies the existence of force, which 

 may consequently be defined as that which produces, or tends to 

 produce, motion in matter ; or alters, or tends to alter, the existing 

 motion of matter. 



The inability shown by a material body of itself to 

 change its condition of rest or of uniform motion is called 

 its inertia. Inertia may also be defined as the capacity of 

 a body to possess momentum. We become aware of the inertia 

 of matter most unpleasantly if we step out of a moving train on 

 to the platform ; while in the train we partook of its motion, on 

 stepping out our feet are brought to rest suddenly, but our 

 bodies, because of their inertia, continue to move with the 

 velocity of the train, with the result that we fall forwards on to 



