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 your hand, you are lifting up your body a little to do it; that is action and reaction. But after you take your hand off the scale the pan springs back up: first it was pushed down, then it springs back to its original position; it does this because of the elasticity of its spring.
Application 23. Explain why basket balls are filled with air; why springs are usually made of steel; why we use rubber bands to hold papers together; why a toy balloon becomes small again when you let the air out.
Inference Exercise
Explain the following, being especially careful not to confuse action and reaction with elasticity:
111. When you want to push your chair back from a table, you push forward against the table.
112. The pans in which candy is cooled must be greased.
113. Good springs make a bed comfortable.
114. Paper clips are made of steel or spring brass.
115. A spring door latch acts by itself if you close the door tightly.
116. On a cold morning, you rub your hands together to warm them.
117. If an electric fan is not fastened in place and has not a heavy base, it will move backward while it is going.
118. Doors with springs on them will close after you.
119. When you jump down on the end of a springboard, it throws you into the air.
120. You move your hands backward to swim forward.
Note. There are really two kinds of elasticity, which have nothing to do with each other. Elasticity of form is the tendency of a thing to go back to its original shape, as rubber does. If you make a dent in rubber, it springs right back to the shape it had before. Elasticity of volume is the tendency of a substance to go back to its original size, as lead does. If you manage to squeeze lead into a smaller space, it will spring right back to the same size as soon as you stop pressing it on all sides. But a dent in lead will stay there; it has little elasticity of form.
Air and water—all liquids, in fact—have a great deal of elasticity of volume, but practically no elasticity of form. They do not tend to keep their shape, but they do tend to fill the same amount of space. Putty and clay likewise have very little elasticity of form; when you change their shape, they stay changed.
Jelly and steel and glass have a great deal of elasticity of form. When you dent them or twist them or in any way change their shape, they go right back to their first shape as soon as they can.
When we imagined a man with an "elastic touch," we were imagining a man who gave everything he touched perfect elasticity of form. It is elasticity of form that most people mean when they talk about elasticity.