Читать книгу Common Science - Carleton Washburne - Страница 3
ОглавлениеGRAVITATION
Section 1. A real place where things weigh nothing and where there is no up or down.
Why is it that the oceans do not flow off the earth?
What is gravity?
What is "down," and what is "up"?
There is a place where nothing has weight; where there is no "up" or "down"; where nothing ever falls; and where, if people were there, they would float about with their heads pointing in all directions. This is not a fairy tale; every word of it is scientifically true. If we had some way of flying straight toward the sun about 160,000 miles, we should really reach this strange place.
Let us pretend that we can do it. Suppose we have built a machine that can fly far out from the earth through space (of course no one has really ever invented such a machine). And since the place is far beyond the air that surrounds the earth, let us imagine that we have fitted out the air-tight cabin of our machine with plenty of air to breathe, and with food and everything we need for living. We shall picture it something like the cabin of an ocean steamer. And let us pretend that we have just arrived at the place where things weigh nothing:
When you try to walk, you glide toward the ceiling of the cabin and do not stop before your head bumps against it. If you push on the ceiling, you float back toward the floor. But you cannot tell whether the floor is above or below, because you have no idea as to which way is up and which way is down.
As a matter of fact there is no up or down. You discover this quickly enough when you try to pour a glass of water. You do not know where to hold the glass or where to hold the pitcher. No matter how you hold them, the water will not pour—point the top of the pitcher toward the ceiling, or the floor, or the wall, it makes no difference. Finally you have to put your hand into the pitcher and pull the water out. It comes. Not a drop runs between your fingers—which way can it run, since there is no down? The big lump of water stays right on your hand. This surprises you so much that you let go of the pitcher. Never mind; the pitcher stays poised in mid-air. But how are you going to get a drink? It does not seem reasonable to try to drink a large lump of water. Yet when you hold the lump to your lips and suck it you can draw the water into your mouth, and it is as wet as ever; then you can force it on down to (or rather toward) your throat with your tongue. Still you have left in your hand a big piece of water that will not flow off. You throw it away, and it sails through the air of the cabin in a straight line until it splashes against the wall. It wets the wall as much as water on the earth would, but it does not run off. It sticks there, like a splash-shaped piece of clear, colorless gelatin.
Suppose that for the sake of experimenting you have brought an elephant along on this trip. You can move under him (or over him—anyway between him and the floor), brace your feet on the floor, and give him a push. (If he happens to step on your toes while you are doing this, you do not mind in the least, because he does not weigh anything, you know.) If you push hard enough to get the elephant started, he rises slowly toward the ceiling. When he objects on the way, and struggles and kicks and tries to get back to the floor, it does not help him at all. His bulky, kicking body floats steadily on till it crashes into the ceiling.
No chairs or beds are needed in this place. You can lie or sit in mid-air, or cling to a fixture on a wall, resting as gently there as a feather might. There is no need to set the table for meals—just lay the dishes with the food on them in space and they stay there. If the top of your cup of chocolate is toward the ceiling, and your plate of food is turned the other way, no harm is done. Your feet may happen to point toward the ceiling, while some one else's point toward the floor, as you sit in mid-air, eating. There is some difficulty in getting the food on the dishes, so probably you do not wish to bother with dishes, after all. Do you want some mashed potatoes? All right, here it is—and the cook jerks the spoon away from the potatoes, leaving them floating before you, ready to eat.
It is literally a topsy-turvy place.
Do you want to know why all this would happen? Here is the reason: There is a great force known as gravitation. It is the pull that everything in the universe has on everything else. The more massive a thing is, the more gravitational pull it has on other objects; but the farther apart things are, the less pull they have on each other.
The earth is very massive, and we live right on its surface; so it pulls us strongly toward it. Therefore we say that we weigh something. And since every time we roll off a bed, for instance, or jump off a chair, the earth pulls us swiftly toward it, we say that the earth is down. "Down" simply means toward the thing that is pulling us. If we were on the surface of the moon, the moon would pull us. "Down" would then be under our feet or toward the center of the moon, and the earth would be seen floating up in the sky. For "up" means away from the thing which is pulling us.
Why water does not flow off the earth. It was because people did not know about gravitation that they laughed at Columbus when he said the earth was round. "Why, if the earth were round," they argued, "the water would all flow off on the other side." They did not know that water flows downhill because the earth is pulling it toward its center by gravitation, and that it does not make the slightest difference on which side of the earth water is, since it is still pulled toward the center.
Why the world does not fall down. And people used to wonder "what held the earth up." The answer, as you can see, is easy. There simply is no up or down in space. The earth cannot fall down, because there is no down to fall to. "Down" merely means toward the earth, and the earth cannot very well fall toward itself, can it? The sun is pulling on it, though; so the earth could fall into the sun, and it would do so, if it were not swinging around the sun so fast. You will see how this keeps it from falling into the sun when you come to the section on centrifugal force.
Why there is a place where things weigh nothing. Now about the place where gravitation has no effect. Since an object near the sun is pulled more by the sun than it is by the earth, and since down here near the earth an object is pulled harder by the earth than by the sun, it is clear that there must be a place between the sun and the earth where their pulls just balance; and where the sun pulls just as hard one way as the earth pulls the other way, things will not fall either way, but will float. The place where the pulls of the sun and the earth are equal is not halfway between the earth and the sun, because the sun is so much larger and pulls so much more powerfully than the earth, that the place where their pulls balance is much nearer the earth than it is to the sun. As a matter of fact, it can be easily calculated that this spot is somewhere near 160,000 miles from the earth.
There are other spots like it between every two stars, and in the center of the earth, and in the center of every other body. You see, in the center of the earth there is just as much of the earth pulling one way as there is pulling the other, so again there is no up or down.
Application 1. Explain why the people on the other side of the earth do not fall off; why you have weight; why rivers run downhill; why the world does not fall down.
Section 2. "Water seeks its own level."
