38 GENERAL SCIENCE 



inversely as the square of the distance between the two 

 bodies and which varies directly as the product of the 

 masses of the two bodies. Newton formulated the law in 

 order to account for the fact that the earth pulls bodies 

 toward it and also to account for the maintenance of the 

 planets and their satellites in their respective orbits. 



We know that objects which are free to fall do fall 

 to the earth, and probably most of us would be willing 

 to accept this without question, because 

 we become so accustomed to this force that 

 the question " why " might never occur to 

 us. It is not easy to imagine this great 

 force. It acts through space, but there 

 are no attachments between the bodies 

 attracted. The apple on the tree is exert- 

 ing a pull on the earth just as the earth is 

 exerting a pull on the apple. Whenever 

 the force which holds the stem of the apple 

 to the tree is less than the pull of the earth, 

 the apple falls to the ground. Why does 

 not the earth seem to fall to the apple ? 

 If two balls, one quite large and the other small, are 

 hung side by side (Figure 29) the small ball will be drawn 

 toward the large one and the supporting cord will no longer 

 hang plumb. 



When we say that the law of gravitation is universal, 

 we simply mean that it applies to all the matter of the 

 universe. It exists between the earth, sun, moon, and 

 stars as well as between the objects which are so near us. 

 Weight. Weight is simply the measure of the force 

 of gravitation on any particular mass or quantity of 

 matter. The quantity of matter in a body is the mass 

 of the body. In the last chapter we learned that the 



