Part 11 (1/2)

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 la.s.so 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 la.s.so 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.

[Ill.u.s.tration: 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...o...b..t. 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 s.p.a.ce, 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 amus.e.m.e.nt 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.

[Ill.u.s.tration: 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.