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.
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, bowl or plate to catch the water spilling
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?
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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
- 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!
- Teaching about air pressure? Check out the Flight or Weather show!
- Teaching about Newton’s laws? Check out the Forces, Friction & Movement workshop!
Get in touch with FizzicsEd to find out how we can work with your class.
Forces, Friction & Movement
Years K to 6
Maximum 30 students
School workshop
60 or 90 minutes
Online Class Available
STEM Full Day Accelerator - Primary
Designed from real classroom experiences, this modular day helps you create consistently effective science learning that directly address the new curriculum with easily accessible and cost-effective materials.
So cool
Glad you liked this!
I love this it was so fun!!!!
Fantastic to hear this!