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Upside Down Water

You don’t have to stand on your head to keep this water up

Australian Curriculum links

  • Science > Chemical Sciences >Year 5 > ACSSU077

You’ll need

  • Drinking glass or rigid plastic cup
  • Postcard or cardboard coaster
  • Water

Try this

  1. Fill the glass 2/3 full with water.
  2. Completely cover the mouth of the glass with the postcard, applying pressure so no air can get in between the mouth of the cup and the cardboard.
  3. While holding the card on with your hand, maintain the pressure and slowly turn the glass upside down. Wait a few seconds and with everything upside down, slowly remove your hand from the cardboard.

What’s happening?

The air around us pushes in every direction, with the pressure of about 1kg per square centimetre – which is approximately the mass of balancing a full milk carton on your thumb nail. However, we are really used to this air pressure because we’ve experienced it all our lives, so we don’t notice it. In this experiment, the pressure of the water pushing down on the postcard is low enough compared to the outside air pressure that the air pushing up on the postcard can hold the water in the glass. Other factors that may cause the postcard to stay on the glass include the surface tension of water and the fact that postcards are slightly bendy.