SOME EFFECTS OF NEWTON'S LAWS 51 



Experiment 43. Using the same ball, roll it twice over the 

 same surface, once slowly and once with speed. Note the dis- 

 tances that it travels. 



Experiment 44. Now take two balls, one very much heavier 

 than the other (e.g. a tennis ball and a bowling ball) ; roll them 

 over the same surface, starting them at the same speed, if pos- 

 sible. Note the distances traveled. 



From these two experiments we see that the momen- 

 tum of moving bodies depends upon two things their 

 mass (quantity of matter) and their speed. Then, in 

 general, we may say that the greater the mass of a body 

 or the faster it moves, the greater is its momentum. 

 The rule is commonly stated as follows : The momentum 

 of a body is equal to the product of its mass multiplied by' 

 its velocity (speed). 



Examples of this law are common. A heavy object 

 is not so easily stopped as a light one moving at the 

 same rate. The faster a train is moving, the more force 

 is exerted by an obstacle which stops it, and the more 

 damage is done. The faster you move in riding a wheel, 

 the farther you can " coast " on a level road. In throw- 

 ing a ball, the boy who can start it at the greatest 

 speed throws it farthest. To test our skill in throwing 

 stones we carefully select one of proper weight, some 

 being so heavy that we cannot start them with much 

 speed, while some are so light that the greatest speed we 

 can give them will not make up for their lack of mass. 



61. Center of Gravity. A body acted upon by force, 

 so as to move in a straight line, may turn over and 

 over in its flight (as a thrown pebble does); but one 

 point within the body moves on in a straight line, as if 



