24 JUNIOR GRADE SCIENCE 



supplies us with a definition of force. Nobody finds any difficulty 

 in understanding the rule so far. But it is not so easy to see the meaning 

 oi tbo words referring to uniform motion in a straight line. An example 

 will make this clear. Consider a ball moving uniformly along ice. 

 After a time the ball comes to rest, and therefore it does not continue 

 in a state of uniform motion. But it moves for a longer time on ice 

 than it would do on a road. The ice is smoother than the road, and 

 there is a connection between the roughness or smoothness and the 

 length of time during which the ball moves. If the ice could be made 

 smoother and smoother, the ball would move for a longer and longer 

 time, and if both the ball and the ice were perfectly smooth, there is 

 no reason why the ball should ever stop. The roughness or friction 

 represents, then, the force which causes the ball to change its state of 

 uniform motion for one of rest. If a body in a state of uniform motion 

 could be placed outside the influence of what Newton has called " im- 

 pressed forces " it would afford us an example of perpetual motion. 

 But because these impressed forces cannot be eliminated perpetual 

 motion is impossible. 



Definition of force. Newton's first law enables force to be defined. 

 Force is that which produces, or tends to produce, motion in matter ; 

 or alters, or tends to alter, the existing motion of matter. It must, 

 however, be clearly understood that by defining force we do not get 

 to know anything more about it. Nobody can tell what force is. All 

 we can know are the effects produced by force. 



The force of gravitation. Experiments and observations made 

 by Newton led him to the conclusion that it was the rule of nature for 

 every material object to attract every other object, and that this force 

 of attraction is proportional to the masses of the bodies ; a large mass 

 exerts a greater force of attraction than a small mass. But the farther 

 these bodies are apart the less is the attraction between them, though 

 it is not less in the proportion of this distance, but hi that of the square 

 of the distance. This diminution of a force according to the inverse 

 proportion of the square of the distance applies to so many cases that 

 it ought to be clearly understood before proceeding. Suppose two 

 bodies of equal mass are one foot away from one another and attract 

 each other with a certain force. If the distance between the masses 

 is doubled, the strength of the attraction between them is only one- 

 quarter of what it was ; for the square of 2 is 2 x 2 =4 and the inverse 

 of 4 is 3;. In the same way, if the bodies are placed three feet apart, 

 the force of attraction is 1 of the original force. Putting Newton's 

 law together it stands thus : Every body in nature attracts every other 

 body with a force directly proportional to the product of their masses 



