CHAPTER II 



FORCE AND ENERGY 



18. Gravity. Many of our most common phenomena 

 are simply changes in the position of bodies. The falling 

 of an apple, the movement of water in waves and tides v 

 the flight of a stone or a bullet through the air, all illustrate 

 this. So do the turning of a magnetic needle toward the 

 poles and the vibrating of a violin string. Now, why 

 does a stone or an apple "fall"? Sir Isaac Newton gave 

 the reason when he said that the earth pulls the falling 

 apple and all other bodies. We do not find it easy to 

 picture to ourselves just how the earth's "pull" is applied. 

 A horse pulling a wagon is attached to the wagon, and the 

 strength of his muscles overcomes the tendency of the 

 wagon to remain at rest. Similarly, an engine is attached 

 to the cars it pulls. But the earth's attraction acts 

 through space, without visible or invisible attachment. 



We can illustrate an attraction similar to the earth's by the 

 action of a magnet upon an iron nail. The magnet is a piece of steel 

 which has been "magnetized," so that it has the power of drawing to 

 itself bodies consisting of steel, iron, nickel, etc. There is no connection 

 between the magnet and the attracted object, yet we know that there 

 is action between them. 



It is just as necessary for us to assume that the apple 

 attracts the earth as that the earth attracts the apple. 

 It is also just as reasonable to suppose that the earth falls 



17 



