18 



FORCE AND ENERGY 



toward the apple as that the apple falls toward the earth. 

 But the distance that the earth moves before they meet 

 must, of course, be very small, owing to the much greater 

 mass of the earth. This earth-pull we call gravity. 



19. Gravitation. The attraction which exists be- 

 tween the earth and bodies near its surface exists also 



r= between all bodies of matter on the earth 

 and between the earth and the sun, moon, 

 and other heavenly bodies. It is called gravi- 

 tation. Gravity is merely a particular case 

 of gravitation. 



The gravitation, or attraction, between two 

 bodies on the earth, as, for example, between 

 two suspended balls (Fig. 14), is not easily 

 observed, because the great attraction of the 

 earth for both of them holds them in a ver- 

 tical position. By means of a celebrated 

 experiment first carried out in the latter part 

 The wo "aii ^ ^ e l^ n century this attraction was made 



attract each visible. A large ball of lead and a small one 



other. 



of copper were suspended side by side, with 

 the result that the copper ball was drawn aside from a 

 vertical position. 



20. Mass and Weight. We need to distinguish be- 

 tween the mass of a body and the weight of a body. New- 

 ton saw that the earth's attraction for a body depends on 

 the quantity of matter in the body, and not upon its kind. 

 A pound of feathers is attracted with the same force as a 

 pound of lead. The quantity of matter in a body is called 



