Common Science Part 11

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.

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.

SECTION 13. _Action and reaction._

How can a bird fly? What makes it stay up in the air?

What makes a gun kick?

Why do you sink when you stop swimming?

Whenever anything moves, it pushes something else in an opposite direction. When you row a boat you can notice this; you see the oars pushing the water backward to push the boat forward. Also, when you shoot a bullet forward you can feel the gun kick backward; or when you pull down hard enough on a bar, your body rises up and you chin yourself. But the law is just as true for things which are not noticeable. When you walk, your feet push back against the earth; and if the earth were not so enormous and you so small, and if no one else were pushing in the opposite direction, you would see the earth spin back a little for each step you took forward, just as the big ball that a performing bear stands on turns backward as the bear tries to walk forward.

[Ill.u.s.tration: FIG. 38. The horse goes forward by pushing backward on the earth with his feet.]

The usual way of saying this is, "Action and reaction are equal and opposite." If you climb a rope, the upward movement of your body is the action; but you have to pull down on the rope to lift your body up. This is the reaction.

Without this law of action and reaction no fish could swim, no steamboat could push its way across the water, no bird could fly, no train or machine of any kind could move forward or backward, no man or animal could walk or crawl. The whole world of living things would be utterly paralyzed.

[Ill.u.s.tration: FIG. 39. As he starts to toss the ball up, will he weigh more or less?]

When _anything_ starts to move, it does so by pushing on something else. When your arms start to move up, they do so by pushing your body down a little. When you swim, you push the water back and down with your arms and legs, and this pushes your body forward and up. When a bird flies up into the air, it pushes its body up by beating the air down with its wings. When an airplane whirs along, its propeller fans the air backward all the time. Street-car tracks are kept shiny by the wheels, which slip a little as they tend to shove the track backward in making the car move forward. Automobile tires wear out in much the same way,--they slip and are worn by friction as they move the earth back in pushing the automobile forward. In fact, if there are loose pebbles or mud on the road, you can see the pebbles or mud fly back, as the wheels of the automobile begin to turn rapidly and give their backward push to the earth beneath.

[Ill.u.s.tration: FIG. 40. Action and reaction are equal; when he pushes forward on the ropes, he pushes backward with equal force on the seat.]

Here are a couple of experiments that will show you action and reaction more clearly:

EXPERIMENT 26. Stand on a platform scale and weigh yourself.

When the beam is exactly balanced, move your hands upward and notice whether you weigh more or less when they _start_ up.

Now move them downward; when they _start_ down, do you weigh more or less? Toss a ball into the air, and watch your weight while you are tossing it. Does your body tend to go up or down while you are making the ball go up?

EXPERIMENT 27. Go out into the yard and sit in a rope swing.

Stop the swing entirely. Keep your feet off the ground all through the experiment. Now try to work yourself up in the swing; that is, make it swing by moving your legs and body and arms, but not by touching the ground. (Try to make it swing forward and backward only; when you try to swing sidewise, the distance between the ropes spoils the experiment.) See if you can figure out why the swing will not move back and forth.

Notice your bodily motions; notice that when half of your body goes forward, half goes back; when you pull back with your hands, you push your body forward. If you watch yourself closely, you will see that every backward motion is exactly balanced by a forward motion of some part of your body.