Upside down water cup experiment | Fizzics Education

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# Upside-down water cup

### You will need:

• One cup of water full to the brim with water.
• One piece of card, larger than the cup rim. You can also use paper.
• An area that you can get wet.

1

Fill your cup with water (try different levels each time!).

Gently place a dry card on top of the cup, making sure there is good contact over the cup rim.

If doing this inside have a tray, bowlÂ or plate to catch the water spilling

2

Carefully turn the cup upside down, keeping upward pressure on the card with your hand.

When the cup is upside down, let go of the card.

Can you explain why the water doesn’t come out?

3

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### Why Does This Happen?

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 the water pushing towards the ground – keeping the water in the cup.

### Variables to test

More on variables here

• Try progressively larger cups. How big can you get? Try a bucket!
• Does it matter which type of liquid is used? Try a variety of liquids of different densities.

### Applications

Knowledge of how pressure works allow engineers to design structures. Airlocks, for example, are used by deep-sea divers leave and enter their submersible vehicles whilst completely underwater. The air inside the airlock is compressed at the same pressure as that of the water outside the airlock. The equal pressure stops the water from entering the airlock. NASA even uses airlocks during their underwater training!

## 4 thoughts on “Upside-down water cup”

1. Hannah Baker says:

So cool

1. Ben Newsome says: