20 MATTER AND ENERGY 



for convenience it is called gravity when spoken of in 

 connection with the earth. 



It is hard to realize the importance of gravity and 

 the part that it plays in our daily lives. Without this 

 force nothing .would stay on earth if it were once moved 

 upward ; a ball thrown into the air would never return ; 

 a locomotive would have no weight upon the rails ; and 

 we could not even walk. 



29. Law of Gravitation. Care must be taken to 

 avoid thinking of gravitation as magnetic force. The 

 two are very different; for while magnetism is shown 

 in only a few substances, gravitation is a common prop- 

 erty of all forms of matter equally. One form of mat- 

 ter can exert as much of this force as another form, and 

 the amount which any body can exert depends only 

 upon the quantity of matter that it contains. It is 

 because the earth contains so much matter that the 

 force of gravity is so strong. 



When two bodies attract each other, the strength of 

 the force depends upon two things the quantity of 

 matter in them and the distance between their centers. 

 The greater the amount of matter, the more they attract 

 each other; the greater the distance, the less they 

 attract. These two facts taken together make up the 

 Laiv of Gravitation. 



30. Weight. Keeping this law in mind, a little 

 thought will show us that the force with which two 

 bodies at the same place will be drawn towards the earth 

 depends only upon the quantities of matter in them. 

 That is, the more matter a body contains, the stronger 



