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Air pushes every way

Air and fluids · level 3, lesson 2 of 6

A question to start

Fill a glass to the brim with water, put a piece of card on top, and turn it upside down over a sink. Let go of the card. The water stays in! What is holding the card up?

The idea

All the air above us, kilometres and kilometres of it, is pulled down by gravity. Its weight presses on everything underneath. This push is air pressure.

Air pressure does not only push down. It pushes in every direction: down, sideways and up, all just as hard.

So under the upside-down glass, the air pushes up on the card. That push is far bigger than the weight of the water in the glass, so the card stays on.

air pushes upair pushessidewayswater

You do not feel this push, because the air and water inside your body push out just as hard. The pushes balance.

Worked examples

1. The card trick. The air pushing up on the card is much stronger than the weight of the water pushing down, so the water stays in.

2. Tilt it. If air gets into the glass, it pushes down on the water from inside, and the water pours out.

3. A squashed can. If you take the air out of a can with a pump, the air outside pushes in on every side, and the can crumples. The air inside was holding it up.

Try it

  1. What is air pressure?
  2. Which ways does air pressure push?
  3. Why do you not feel the air pressing on you?
  4. Why does the water pour out if air gets into the glass?
Answers
  1. The push of the air, from the weight of all the air above.
  2. In every direction: down, sideways and up.
  3. The air and water inside your body push out just as hard.
  4. The air inside pushes down on the water as hard as the air outside pushes up on the card.

Common mistake

"The water sticks to the card." It is the air outside pushing up that holds the card on. Without air, in a vacuum, the trick does not work.

Where it leads