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
swiftly forward, up, and on around, bringing it down back of
you (Fig. 36). Swing it around this way swiftly and evenly
several times, finally stopping at the beginning of the up
swing.
It is centrifugal force that keeps the water in the pail. It depends
entirely on inertia. You see, while the pail is swinging upward
rapidly, the water is moving up and tends by its inertia to keep right
on moving in the same upward direction. Before you get it over your
head, the tendency of the water to keep on going up is so strong that
it pulls on your arm and hand and presses against the bottom of the
pail above it. Its tendency to go on up is stronger than the downward
pull of gravity. As you swing the pail on backward, the water of
course has to move backward, too; so now it tends to keep on moving
backward; and when the pail is starting down behind you, the water is
tending to fly out in the backward direction in which it has just been
going. Therefore it still pushes against the bottom of the pail and
pulls away from your shoulder, which is in the center of the circle
about which the pail is moving. By the time you have swung the pail
on down, the water in it tends to keep going down, and it is still
pulling away from your shoulder and pressing against the bottom of the
pail.
[Illustration: FIG. 36. Why doesn't the water spill out?]
In this way, during every instant the water tends to keep going in the
direction in which it was going just the instant before. The result
is that the water keeps pulling away from your shoulder as long as you
keep swinging it around.
_All whirling things tend to fly away from the center about which they
are turning._ This is the law of centrifugal force. The earth, for
example, as it swings around the sun, tends to fly away from the
center of its orbit. This tendency of the earth--its centrifugal
force--keeps it from being drawn into the sun by the powerful pull of
the sun's gravitation. At the same time it is this gravitation of the
sun that keeps the earth from flying off into space, where we should
all be frozen to icicles and lost in everlasting night. For if the
sun's pull stopped, the earth would fly off as does a stone whirled
from the end of a string, when you let go of the string.
The moon, in like manner, would fly away from the earth and sun if
_gravitation_ stopped pulling it, but it would crash into us if its
_centrifugal force_ did not keep it at a safe distance.
Have you ever sat on a spinning platform, sometimes called "the social
whirl," in an amusement park, and tried to stay on as it spun faster
and faster? It is centrifugal force that makes you slide away from the
center and off at the edge.
[Illustration: FIG. 37. An automobile race. Notice how the track is
banked to keep the cars from overturning on the curves.]
HOW CREAM IS SEPARATED FROM MILK BY CENTRIFUGAL FORCE. The heavier
things are, the harder they are thrown out by centrifugal force. Milk
is heavier than cream, as you know from the fact that cream rises
and floats on top of the milk. So when milk is put into a centrifugal
separator, a machine that whirls it around very rapidly, the milk is
thrown to the outside harder than the cream, and the cream therefore
stays nearer the middle. As the bowl of the machine whirls faster, the
milk is thrown so hard against the outside that it flattens out and
rises up the sides of the bowl. Thus you have a large hollow cylinder
of milk on the outside against the wall of the bowl, while the
whirling cream forms a smaller cylinder inside the cylinder of
milk. By putting a spout on the machine so that it reaches the inner
cylinder, the cream can be drawn off, while a spout not put in so far
will draw off the milk.
WHY A SPINNING TOP STANDS ON ITS POINT. When a top spins, all the
particles of wood of which the top is made are thrown out and away
from the center of the top, or rather they _tend_ to go out and away.
And the pull of these particles out from the center is stronger than
the pull of gravitation on the edges of the top to make it tip over;
so it stands upright while it spins. Spin a top and see how this is.
_APPLICATION 21._ Explain how a motor cyclist can ride on an
almost perpendicular wall in a circular race track. Explain
how the earth keeps away from the sun, which is always
powerfully pulling the earth toward it.
INFERENCE EXERCISE
Explain the following:
91. As you tighten a screw it becomes harder to turn.
92. There is a process for partly drying food by whirling it
rapidly in a perforated cylinder.
93. It is easier to climb mountains in hobnailed shoes than in
smooth-soled ones.
94. When you bore a hole with a brace and bit, the hand that
turns the brace goes around a circle many times as large as
the hole that is being bored.
95. The hands of some persons become red and slightly swollen
if they swing them while taking a long walk.
96. A flywheel keeps an engine going between the strokes of
the piston.
97. In dry parts of the country farmers break up the surface
of the soil frequently, as less water comes up to the surface
through pulverized soil than would come through the fine pores
of caked earth.
98. After you have apparently cleaned a grease spot out of
a suit it often reappears when you have worn the suit a few
days.
99. Mud flies up from the back wheel of a boy's bicycle when
he rides along a wet street.
100. A typewriter key goes down less than an inch, yet the
type bar goes up nearly 5 inches.