Cool coin launcher

17 October 2011

Part of the show Why Is Ice Slippery?

freezer-coin.jpg

Produce a coin launcher powered by your freezer

Ingredients

An empty fizzy drinks bottle	A large coin
A freezer	Some Tissue

Instructions

Put the lid on your bottle, and then put it in the freezer for half an hour or so.

Have a look at it has anything changed?

Open the bottle while it is still in the freezer so it doesn't warm up.

Cover the coin in a layer of tissue, and then make it wet.

Place the coin on the bottle with the tissue between the coin and the top of the bottle.

Leave the bottle for another half an hour until the tissue freezes

Take the bottle out of the freezer point it away from you and warm it up using your hands.

Result

You should find that the closed bottle shrinks as it cools down.

When you take the bottle out of the freezer and warm it up the coin should pop off. If it doesn't the tissue might not have made a good seal so you might want to try another one.

Explanation

When you cool air or any other gas it shrinks, so the closed bottle is crushed by the air outside it.


The pressure is the same inside and out.	As it cools the pressure inside drops but the pressure outside is still the same, as more air can enter the freezer.	The pressure outside crushes the bottle until the pressures equalise

If the bottle is open as the air shrinks extra air is pushed into the bottle.Then the wet coin seals the top of the bottle as it freezes.

This means that when you heat up the air again it pushes outwards harder and harder until the pressure on the bottom of the coin is large enough to break the ice around it. As the ice breaks the coin is pushed upwards and flies away from the bottle.


The open neck means more air can enter the bottle, to balance the pressures.	There is extra air in the bottle now, and the coin holds it there, so as the temperature increases the pressure builds up.	Eventually the ice breaks and the pressure difference pushes the coin off the top of the bottle.

Why does air expand when you heat it up?

Air is made up of small particles which are bouncing off one another and the walls of the container they are in. These collisions push outwards and cause a gas to take up space. A hotter gas has more energy so the particles move faster and bash into the walls faster and more often so the gas pushes harder and takes up more space. This pressure pushes the coin off the bottle.

Try changing the temperature of the gas in the balloon (courtesy of daveansell.co.uk)