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Upside down water cup experiment

You will need:

 
  • 1 styrofoam cup of water, full to the brim
  • 1 piece of card, larger than the cup rim (you could use paper too!)
  • An area where you can get wet!

    Copyright notice

Instructions

 
  1. Make sure your cup is completely full, i.e. about to spill over the side.
     
  2. Gently place a dry card on top of the cup... making sure there is good contact over the cup rim.
     
  3. Carefully turn the cup upside down, keeping upward pressure on the card

    Air pressure demonstration
     
  4. Can you explain why the water doesn't come out?

Why does this work?


Air molecules are constantly pushing into things, in every direction imaginable. In other words, air has pressure. If you look at your thumbnail, you have the equivalent of around 1 kg of weight pushing down on that body part alone!

Inside the cup there was no air, so the weight inside the cup was coming only from the water. Depending on the size of your cup, the weight of the water may have been around 250g. The air below the card was pushing up into the card. The upwards air pressure was much greater than that of the pressure of water pushing towards the ground - keeping the water in the cup.

Air locks are used by deep sea divers leave their submersible vehicles. The air inside the air lock is compressed at the same pressure as that of the water outside the air lock.

The equal pressure stops the water from entering the air lock.
NASA even uses airlocks during their underwater training!
 
Other air pressure based science experiments
 
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