ss. 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