英文科學(xué)讀本 第四冊(cè)·Lesson 28 Buoyancy of Liquids

Lesson 28 Buoyancy of Liquids

We have seen that water and all liquids press downwards, sideways, upwards—in fact in every direction, said Mr. Wilson, "and that at the same depth the pressure is equal in every direction. I want now to direct your thoughts towards the pressure of liquids in one direction only—upwards. There is much to be learned from this.

I have here a small square block of glass. I will attach this piece of silk thread to it. Now Fred shall come to the front and hold it suspended by the thread. He will tell us at once that he feels a downward pressure as he holds it. What causes that downward pressure?

It is the pressure of gravity, sir, or the weight of the block itself.

Quite right. Now I want you to hold the thread in such a position as to let the glass block dip into this bowl of water. Tell us what you notice now, Fred.

I can feel the water pressing the glass up, sir, said Fred. "It does not seem to weigh so heavy now."

Well, suppose we try to find out what all this means. We will begin by weighing the piece of glass. It weighs exactly three ounces. This means that when we hang it from the thread it presses down with a weight of three ounces. Now I am going to suspend the piece of glass from the scale-pan by means of the thread. We still require the three—ounce weight in the other scale to balance it. It still weighs three ounces. The two arms of the balance are horizontal.

Now place the bowl under it, and let the glass dip into the water. What do you see?

The scales do not balance now. The weight scale sinks.

What does this prove?

It proves that the glass does not weigh three ounces now, sir.

Quite right; let us see what it does weigh. I will take out one of the ounce weights, and now, you see, the arms of the balance once more rest in the horizontal position. The glass, we know, is pressing downwards with a weight of three ounces, but the water is pressing upwards, and it is this upward pressure of the water which seems to rob the glass of one-third of its weight.

Suppose, before we go any further, we try the same experiment with a piece of brick and a piece of sulphur. Each of these substances loses half its weight when suspended in the water.

Now if I detach the glass, the brick, and the sulphur from the thread, they will all sink to the bottom of the water. Why do they sink?

The downward pressure of their weight is greater than the upward pressure of the water.

We have learned then that all solid bodies weigh less in water than in air, but some, such as glass, sulphur, brick, and of course such bodies as stone and metal, sink to the bottom if unsupported. We call these heavy bodies.