What makes a ball bounce?
How does a springboard help you dive?
Why are automobile and bicycle tires filled with air?
Suppose there were a man who was perfectly elastic, and who made
everything he touched perfectly elastic. Fortunately there is no such
person, but suppose an elastic man _did_ exist:
He walks with a spring and a bound; his feet bounce up like rubber
balls each time they strike the earth; his legs snap back into place
after each step as if pulled by a spring. If he stumbles and falls to
the ground, he bounces back up into the air without a scar. (You see,
his skin springs back into shape even if it is scratched, so that a
scratch instantly heals.) And he bounces on and on forever without
stopping.
Suppose you, seeing his plight, try to stop him. Since we are
pretending that he makes everything he touches elastic, the instant
you touch him you bounce helplessly away in the opposite direction.
You may think your clothes will be wrinkled by all this bouncing
about, but since we are imagining that you have caught the elastic
touch from the elastic man, your clothes which touch you likewise
become perfectly elastic. So no matter how mussed they get, they
promptly straighten out again to the condition they were in when you
touched the elastic man.
If you notice that your shoe lace was untied just before you became
elastic, and you now try to tie it and tuck it in, you find it most
unmanageable. It insists upon flying out of your shoe and springing
untied again.
Perhaps your hair was mussed before you became elastic. Now it is
impossible to comb it straight; each hair springs back like a fine
steel wire.
If you take a handkerchief from your pocket to wipe your perspiring
brow, you find that it does not stay unfolded. As soon as it is spread
out on your hand, it snaps back to the shape and the folds it had
while in your pocket.
Suppose you bounce up into an automobile for a ride. The automobile,
now being made elastic by your magic touch, bounds up into the air
at the first bump it strikes, and thereafter it goes hopping down the
street in a most distressing manner, bouncing off the ground like a
rubber ball each time it comes down. And each time it bumps you are
thrown off the seat into the air.
You find it hard to stay in any new position. Your body always
tends to snap back to the position you were in when you first became
elastic. If you touch a trotting horse and it becomes elastic,
the poor animal finds that his legs always straighten out to their
trotting position, whether he wants to walk or stand still or lie
down.
Imagine the plight of a boy pitching a ball, or some one yawning and
stretching, or a clown turning a somersault, if you touch each of
these just in the act and make him elastic. Their bodies always tend
to snap back to these positions. Whenever the clown wants to rest, he
has to get in the somersault position. The boy pitcher sleeps in the
position of "winding up" to throw the ball. The one who was yawning
and stretching has to be always on the alert, because the instant he
stops holding himself in some other position, his mouth flies open,
his arms fly out, and every one thinks he is bored to death.
You might touch the clay that a sculptor is molding and make it
elastic. The sculptor can mold all he pleases, but the clay is like
rubber and always returns at once to its original shape.
If you make a tree elastic when a man is chopping it down, his ax
bounces back from the tree with such force as nearly to knock him
over, and no amount of chopping makes so much as a lasting dent in the
tree.
Suppose you step in some mud. The mud does not stick to your shoes. It
bends down under your weight, but springs back to form again as soon
as your weight is removed.
And if you try to spread some elastic butter on bread, nothing will
make the butter stay spread. The instant you remove your knife, the
butter rolls up again into the same kind of lump it was in before.
As for chewing your bread, you might as well try to chew a rubber
band. You force your jaws open, and they snap back on the bread all
right; then they spring open again, and snap back and keep this up
automatically until you make them stop. But for all this vigorous
chewing your bread looks as if it had never been touched by a tooth.
Sewing is about as difficult. The thread springs into a coil in the
shape of the spool. No hem stays turned; the cloth you try to sew
springs into its original folds in a most exasperating manner.
On the whole, a perfectly elastic world would be a hopeless one to
live in.
_Elasticity is the tendency of a thing to go back to its original
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 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.
CHAPTER FOUR
HEAT