Air presses against any object that it touches; wind is moving air and it presses anything that lies in its path. Air which seems gentle when it is calm can become a tornado(龙卷风) that sweeps buildings and whole towns away.
But even when it is calm, air presses, and it presses from all directions. The following experiment shows that this statement is true.
Get an empty soup can from which the top has been removed. Make sure that the rim(边缘) is smooth.
Fill the can with water. Place a piece of cardboard over the top of the can and hold it there. Quickly turn the can from side to side to see what happens.
No matter how you turn the soup can, the cardboard will stay in position and the water will stay inside. Air is pressing against the cardboard from without, and water is pressing against it from within. Because the air is pressing harder than the water, the cardboard stays in place and the water stays inside the can.
Because air presses, we say that air exerts(施加) pressure. Air exerts pressure because it has weight.
If you were to dive into a deep pool of water, the weight of the water would push against your whole body. The pressure caused by the weight of the water would work in all directions—up and down and on every side.
So is in air. You do not feel the weight of air against your body because the pressure it exerts on one side of you is balanced by the pressure on the opposite side. And don’t forget! The air inside you has weight. So it, too exerts pressure.
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Choose the best answer:
1. The experiment given in the passage proved __________ .
A. air is all around us though it is invisible
B. air becomes a wind when it moves
C. air exerts pressure because it has weight
D. air presses and it presses from all directions
2. The writer tells you to make sure the rim of the can is smooth in case __________ .
A. the water falls out B. the air fills in
C. the cardboard fails D. it might cut someone
3. Which of the following prevents the water falling out of the can?
A. The pressure of air. B. The pressure of water.
C. The pressure of both air and water. D. None of the above.
4. You don’t feel the weight of air pushing against your body because the pressure it exerts works __________ .
A. inside you B. on one side of you
C. on the other side of you D. on every side of you
参考答案：1.D 2.D 3.A 4.D
