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This is a hold-up

By Mike, 4 October 2013 Activity

Written by Justin McGuire

This activity may splash some water around, so try it over a sink or outside.

You will need

  • Small drinking glass
  • Piece of cardboard large enough to cover the mouth of the glass
  • Water
  • Food colouring (optional)
Inverted glass containing green water, with a card pressed beneath to keep the water in.

Hold the cardboard in place while you turn the glass over. Let go of the cardboard. What happens?

What to do

  1. Fill the glass until the water is almost overflowing.
  2. If you wish, add a few drops of food colouring to the water.
  3. Place the cardboard on top of the glass. Don’t worry if a bit of water trickles down the side of the glass.
  4. Hold the glass in mid-air.
  5. While holding the card in place, turn the glass upside down.
  6. Let go of the cardboard. What happens?

What’s happening?

You should find the cardboard remains stuck to the bottom of the glass.

You are seeing air pressure in action. Although the weight of the water is pushing down on the cardboard, the air pressure presses up on the cardboard, holding it in place.

Air around us pushes in on us all the time. This is called atmospheric pressure. Atmospheric pressure varies depending on altitude and weather conditions, but normally the air around us pushes on things with a pressure of around 1 kg per square cm.

There are two forces to consider here: pushing down from the weight of the water, and pushing up from the pressure of the surrounding atmosphere. As long as the force from the air pressure is greater than the weight of the water, the cardboard is pushed up against the glass.

We can work out exactly how much force the cardboard feels from air pressure. The mouth of our glass was 6.3 cm wide, so (using the formula for the area of a circle) the air pressure was pushing up on an area of 31 square cm. Since air pressure is normally around 1 kg per square cm, the upwards force of the air was equivalent to the weight of 31 kg. The water in the glass would have weighed around 250 g, so the air pressure easily held the card in place.

Simple diagram of the inverted glass, with big arrows pointing up to indicate force of air pressure and small arrows inside the glass pointing down indicating the water's weight.

The weight of the water pressing down on the cardboard is less than the force from the air pressure pushing up from below, so the cardboard is held against the glass.

Applications

When you stick a suction cup to something, you force the air out from under the cup, so the air pressure pushing in on the cup holds it in place.

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