Weightless Water

25 May 2008

Ingredients

A cup A plastic cup Scissors A pair of nail scissors

Instructions

Hole in the cupUse the nail scissors to cut a small hole in the bottom of the cup using the nail scissors, it should be a bit smaller than your finger.

Put your finger over the hole and fill the cup with water.

Stand outside, or somewhere that doesn't mind getting wet - if you want to do the experiment twice with the same cup, stand over something soft.

rest the cup on the finger over the hole, and stabilise it with your other finger.

Drop the cup by pulling your finger downwards as fast as possible - this means that the cup will be released very smoothly.

Do you think that the water will fall out of the cup?

Result

You should find that the water stays inside the cup even though there is a big hole in the bottom - until it hits the ground anyway.

The cup dropping
Dropping the cup The same thing, but with the camera dropping with the cup The cup half way down - the water stops flowing

Explanation

On earth, if something isn't being held up it will accelerate downwards due to gravity. So if you hold onto a cup with a hole in it, the water will be pulled downwards through the hole and end up on the floor.

What happens when you drop the cup?

Galileo worked out that if you just let something fall, it will accelerate towards the ground at the same rate whatever it is. Its speed will increase by 9.81 metres per second every second. This is true of both the water and the cup, separately at the same time they would both hit the ground at the same time (if we can ignore air resistance)

Cup and water falling under gravity

Dropping the cup
If you dropped some water and a cup they would both fall at the same rate, and so hit the floor at the same time. So of course if you dropped some water in a cup, they will both fall at the same rate and so they will stay together.

This means that if you drop them both together they will both fall at the same speed and so the water won't fall out.

What has this got to do with weightlessness and space ships?

On the international space station there is roughly 90% of the gravity on the surface of Earth, but the astronauts float about. This is because when something is in orbit, it is falling all the time just like your cup. And because the space station and the astronauts are both falling at the same rate they don't move relative to one another and so it seems like they have not weight. So you have made the water act as if it were weightless.

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