Why is it that if you push a miniature auto rapidly, it will
go straight?
Why does the earth never stop moving?
When you jerk a piece of paper from under an inkwell, why does
the inkwell stay still?
When you are riding in a car and the car stops suddenly, you are
thrown forward; your body tends to keep moving in the direction in
which the car was going. When a car starts suddenly, you are thrown
backward; your body tends to stay where it was before the car started.
When an automobile bumps into anything, the people in the front seat
are often thrown forward through the wind shield and are badly cut;
their bodies keep on going in the direction in which the automobile
was going.
When you jump off a moving street car, you have to run along in the
direction the car was going or you fall down; your body tries to keep
going in the same direction it was moving, and if your feet do not
keep up, you topple forward.
Generally we think that it takes force to start things to move, but
that they will stop of their own accord. This is not true. It takes
just as much force to stop a thing as it does to start it, and what
usually does the stopping is friction.
When you shoot a stone in a sling shot, the contracting rubber pulls
the stone forward very rapidly. The stone has been started and
it would go on and never stop if nothing interfered with it. For
instance, if you should go away off in space--say halfway between here
and a star--and shoot a stone from a sling shot, that stone would keep
on going as fast as it was going when it left your sling shot, forever
and ever, without stopping, unless it bumped into a star or something.
On earth the reason it stops after a while is that it is bumping into
something all the time--into the particles of air while it is in the
air, and finally against the earth when it is pulled to the ground by
gravity.
If you threw a ball on the moon, the person who caught it would have
to have a very thick mitt to protect his hand, and it would never
be safe to catch a batted fly. For there is no air on the moon, and
therefore nothing would slow the ball down until it hit something; and
it would be going as hard and fast when it struck the hand of the one
who caught it as when it left your hand or the bat.
[Illustration: FIG. 34. When the paper is jerked out, the glass of
water does not move.]
TRY THESE EXPERIMENTS:
EXPERIMENT 23. Fill a glass almost to the brim with water.
Lay a smooth piece of writing paper 10 or 11 inches long on a
smooth table, placing it near the edge of the table. Set the
glass of water on the paper near its inner edge (Fig. 34).
Take hold of the edge of the paper that is near the edge of
the table. Move your hand a little toward the glass so that
the paper is somewhat bent. Then, keeping your hand near the
level of the table, suddenly jerk the paper out from under the
glass. If you give a quick enough jerk and keep your hand near
the level of the table, not a drop of water will spill and the
glass will stay almost exactly where it was.
This is because the glass of water has inertia. It was standing still,
and so it tends to remain standing still. Your jerk was so sudden that
there was not time to overcome the inertia of the glass of water; so
it stayed where it was.
EXPERIMENT 24. Have a boy on roller skates skate down the hall
or sidewalk toward you and have him begin to coast as he comes
near. When he reaches you, put out your arm and try to stop
him. Notice how much force it takes to stop him in spite of
the fact that he is no longer pushing himself along.
Now let the boy skate toward you again, coasting as before;
but this time have him swing himself around a corner by taking
hold of you as he passes. Notice how much force it takes just
to change the direction in which he is moving.
[Illustration: FIG. 35. When a boy is moving rapidly, it takes force to
change the direction of his motion.]
You see the boy's inertia makes him tend to keep going straight ahead
at the same speed; it resists any change either in the speed or the
direction of his motion. So it takes a good deal of force either to
stop him or to turn him.
If, on the other hand, _you_ had no inertia, you could neither have
stopped him nor turned him; he would have swept you right along with
him. It was because inertia made you tend to remain still, that you
could overcome part of his inertia. At the same time he overcame part
of your inertia, for he made you move a little.
Inertia is the tendency of a thing to keep on going forever in the
same direction if once it is started, or to stand still forever unless
something starts it. If moving things did not have inertia (if they
did not tend to keep right on moving in the same direction forever or
until _something_ changed their motion), you could not throw a ball;
the second you let go of it, it would stop and fall to the ground.
You could not shoot a bullet any distance; as soon as the gases of the
gunpowder had stopped pushing against it, it would stop dead and fall.
There would be no need of brakes on trains or automobiles; the instant
the steam or gasoline was shut off, the train or auto would come to a
dead stop. But you would not be jerked in the least by the stopping,
because as soon as the automobile or train stopped, your body too
would stop moving forward. Your automobile could even crash into a
building without your being jarred. For when the machine came to a
sudden stop, you would not be thrown forward at all, but would sit
calmly in the undamaged automobile.
If you sat in a swing and some one ran under you, you would keep going
up till he let go, and then you would be pulled down by gravity just
as you now are. But just as soon as the swing was straight up and down
you would stop; there would be no inertia to make you keep on swinging
back and up.
If the inertia of moving things stopped, the clocks would no longer
run, the pendulums would no longer swing, nor the balance wheels turn;
nothing could be thrown; it would be impossible to jump; there would
cease to be waves on the ocean; and the moon would come tumbling to
the earth. The earth would stop spinning; so there would be no change
from day to night; and it would stop swinging about in its orbit and
start on a rush toward the sun.
But there is always inertia. And all things everywhere and all the
time tend to remain stock still if they are still, until some force
makes them move; and all things that are moving tend to keep on moving
at the same speed and in the same direction, until something stops
them or turns them in another direction.
_APPLICATION 20._ Explain why you should face forward when
alighting from a street car; why a croquet ball keeps rolling
after you hit it; why you feel a jolt when you jump down from
a high place.
INFERENCE EXERCISE
Explain the following:
81. It is much easier to erase charcoal drawings than
water-color paintings.
82. When an elevator starts down suddenly you feel lighter for
a moment, while if it starts up quickly you feel heavier.
83. You can draw a nail with a claw hammer when you could not
possibly pull it with your hand even if you could get hold of
it.
84. When an automobile bumps into anything, the people in the
front seat are often thrown forward through the wind shield.
85. Certain weighted dolls will rise and stand upright, no
matter in what position you lay them down.
86. Some automobile tires have little rubber cups all over
them which are supposed to make the tires cling to the
pavement and thus prevent skidding.
87. It is hard to move beds and bureaus which have no castors
or gliders.
88. When you jump off a moving street car, you lean back.
89. All water flows toward the oceans sooner or later.
90. You can skate on ice, but not on a sidewalk, with ice
skates.