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.

SECTION 12. _Centrifugal force._

Why does not the moon fall down to the earth?

Why will a lasso go so far after it is whirled?

Why does a top stand on its point while it is spinning?

If centrifugal force suddenly stopped acting, you would at first not
notice any change. But if you happened to get into an automobile and
rode down a muddy street, you would be delighted to find that the mud
did not fly up from the wheels as you sped along. And when you went
around a slippery corner, your automobile would not skid in the least.

If a dog came out of a pool of water and shook himself while
centrifugal force was not acting, the water, instead of flying off in
every direction, would merely drip down to the ground as if the dog
were not shaking himself at all. A cowboy would find that he could no
longer throw his lasso by whirling it around his head. A boy trying to
spin his top would discover that the top would not stand on its point
while spinning, any better than when it was not spinning.

These are little things, however. Most people would be quite
unconscious of any change for some time. _Then_, as night came on and
the full moon rose, it would look as if it were growing larger and
larger. It would seem slowly to swell and swell until it filled the
whole sky. Then with a stupendous crash the moon would collide with
the earth. Every one would be instantly killed. And it would be lucky
for them that they were; for if any people survived the shock of the
awful collision, they would be roasted to death by the heat produced
by the striking together of the earth and the moon. Moreover, the
earth would be whirled swiftly toward the sun, and a little later the
charred earth would be swept into the sun's vast, tempestuous flames.

When we were talking about inertia, we said that if there were no
inertia, the moon would tumble down to the earth and the earth, too,
would fall into the sun. That was because if there were no inertia
there would be no centrifugal force. For centrifugal force is not
really a force at all, but it is one form of inertia--the inertia of
whirling things. Do this experiment:

EXPERIMENT 25. Hold a pail half full of water in one hand.
Swing it back and forth a couple of times; then swing it