Conservation of Energy 85 



Elasticity is the tendency of a thing to go back to its orig- 

 inal shape or size whenever it is forced into a different 

 shape or size. 



A thing does not have to be soft to be elastic. Steel 

 is very elastic; that is why good springs are almost 

 always made of steel. Glass is elastic ; you know how 

 you can bounce a glass marble. Rubber is elastic, 

 too. Air is elastic in a different way ; it does not go 

 back to its original shape, since it has no shape, but if 

 it has been compressed and the pressure is removed it 

 immediately expands again ; so a football or any such 

 thing filled with air is decidedly elastic. That is why 

 automobile and bicycle tires are filled with air; it 

 makes the best possible " springs." 



Balls bounce because they are elastic. When a ball 

 strikes the ground, it is pushed out of shape. Since it 

 is elastic it tries immediately to come back to its former 

 shape, and so pushes out against the ground. This 

 gives it such a push upward that it flies back to your 

 hand. 



Sometimes people confuse elasticity with action and 

 reaction. But the differences between them are very- 

 clear. Action and reaction happen at the same time ; 

 your body goes up at the same time that you pull down 

 on a bar to chin yourself; while in elasticity a thing 

 moves first one way, then the other ; you throw a ball 

 down, then it comes back up to you. Another difference 

 is that in action and reaction one thing moves one way 

 and another thing is pushed the other way; while 

 in elasticity the same thing moves first one way, then 

 the other. If you press down on a spring scale with 



