MATTER AND ENERGY 



67 



on its surface. This force is what gives objects weight; 

 it causes objects to fall toward the earth. When a ball 

 is thrown up, gravity stops it and draws it back to the 

 earth's surface. It was Sir Isaac Newton who first 



explained why unattached objects fall. 



The earth's attraction can be illus- 

 trated by the use of a magnet and an 

 iron nail. A magnet is a piece of steel 

 with its molecules so arranged that it 

 can draw small pieces of iron to itself, 

 and yet there is no kind of connection 

 between the magnet and the piece of 

 iron. When the nail touches the mag- 

 net it will adhere to the magnet much as 

 an object is held to the earth by gravity. 



All bodies the sun, moon, stars, 

 and even apples, balls, and stones 

 have the power of attracting other 

 bodies toward them. This general force 



1.111. e 



of attraction which bodies have for one 



another is called gravitation. Gravitation can be 



Two BALLS 



influence 



tation. 



of gravi- 



illus- 



trated by suspending with long strings a large lead ball 

 near a small brass ball. The brass ball will be drawn 

 aside from a vertical position, or the distance between 

 the strings near the balls will be found to be less than it 

 is at the point of suspension. Gravitation is a property 

 of matter, and it is the force of gravitation which holds 

 the earth and all the other heavenly bodies in position. 

 50. Energy. We cannot think of redness or beauty 

 apart from some object or without having some object 

 for their cause. So it is with energy. We cannot think 

 of it except in connection with some material thing. 

 So we define it thus: The energy of a body is its capacity 



