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
gla