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Get in a spin: Sophie Duncan looks at a Newton's third law of motion

What is this wonderful object? Is it a) a glass-making globe, b) an early steam engine or c) a fumigator? The answer is b) an early steam engine.

The aeolipile was invented in Greece in the 1st century BC, and is believed to be the first steam engine. It was designed by Hero of Alexandria but was not employed in any practical way, being seen as an amusing toy. The sphere contained water, which was heated. The steam escaped through the directional nozzles, and the sphere rotated. Although Hero did not know it, the aeolipile was a great example of Newton's third law of motion - for every action there is an equal and opposite reaction.

Although it is possible to make an aeolipile, steam and hot metal do not make for a safe classroom. However, it is possible to test out Newton's third law in the following experiment.

This experiment makes use of an empty milk carton, string and water. Open out the flaps of the milk carton and rinse thoroughly. Make a small mark on each of the four sides in the bottom right-hand corner. Now make holes where the marks are, all the same size. Block each hole with tape. Attach a piece of string to the top of the carton so, once suspended, it can rotate freely. Fill the carton with water and remove the tape. You should find that the carton rotates until the water has gone. (Depending on how you have suspended the carton you may have stored energy in the twisted string, which will be released, causing the carton to move in the opposite direction.) To keep the carton rotating continuously, suspend it under a running tap.

Sophie Duncan is project manager for science at the BBC www.bbc.co.uk

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