英文科學讀本 第四冊·Lesson 12 Gravity

Lesson 12 Gravity

We know that if we let a ball, a stone, or a body of any kind drop from our hands, it must come to the ground, said Mr. Wilson. "Now which of you has ever seen a balloon floating in the air? You all have. Very well. Perhaps one of you may have seen the men in the balloon throwing something out?"

I have seen them throw the sand out to lighten the balloon, said Willie.

Quite true, said Mr. Wilson, "and what became of the sand?"

It fell to the ground.

Yes, Willie, all bodies fall if unsupported, and they always fall in one direction—downwards, towards the ground. What do we call the direction opposite to this?"

Upwards. 'Up' means towards the sky; 'down' means towards the ground. I can throw a ball upwards to a certain height, but it must fall downwards to the ground again.

Now I want you to think for a few minutes about this earth on which we live, continued Mr. Wilson. "You know that it is a great ball, and that you and I and everybody live and move about on the outside of the ball. I will draw a circle to represent the earth. Suppose we wish to show some very tall building, towering up into the sky; I must represent it by a line drawn perpendicular to the earth, that is, perpendicular to the circle itself, without troubling whether it looks upright on the board.

But we must not forget that other people, in other parts of the world, have high buildings too. Suppose we draw some more. You see they all point in different directions, but they are all perpendicular to the circle, which represents the earth. Now watch what I do next. I will draw a straight line through each of these perpendicular towers, and produce them through the circle. What do you see now?"

Every one of these lines passes through the center of the circle, sir, said Fred.

Quite right, but the tops of these towers all point towards the sky, and the bottoms of them are all on the ground. In other words, the tops point up, and the bottoms down. Now think once more about the man in the balloon. Let him drop another stone, while you watch it fall. We will draw a line to show the direction of the falling stone.

Now produce this line. What do you see?

This line too passes through the centre, said Fred.

Now I think we can understand, continued Mr. Wilson, "that 'up' always means towards the sky; 'down' always means towards the center of the earth.

So then we have learned at last that all bodies, when unsupported, fall downwards, and that means towards the center of the earth. But why should they fall, and fall always in this one direction? They fall because the earth attracts or draws them towards itself. There is a force which acts from the center of the earth, and draws all other bodies towards that spot. We call this force gravity. It is the force of gravity which causes the feeling of downward pressure when we hold different bodies in the hand. The earth is trying to draw them downwards towards itself. We say they are heavy. It is the force of gravity which gives all substances weight. The very word gravity means weight.

I daresay you are wondering, continued Mr. Wilson, "why the earth should draw some bodies more than others. It is because the earth attracts bodies in proportion to the amount of matter which they contain, and some bodies, as we have seen, contain more matter than others."