Science corner

26th September 2003 at 01:00
Sophie Duncan finds the centre of gravity

It is good fun to work out how to balance things. These experiments will help you explore the centre of gravity.

For the first experiment your students will need a long stick, such as a metre rule. Ask the students to balance this on their two index fingers, making sure that one end is poking out further than the other. Then ask them to move their fingers slowly together.

Before they do the experiment discuss what they expect to happen. No matter how many times they do it, their fingers move together to the mid-point of the stick, where it is perfectly balanced.

There is greater friction on the hand that supports most of the weight - the one nearest the centre of gravity. As each hand moves, the hand nearest the centre of gravity changes. They finally come to rest at the centre of gravity.

Challenge your students to find the centre of gravity of a flat object, such as a cardboard map of the UK. How would they find the point on the map which would enable them to perfectly balance it on their finger?

Although trial and error will find the correct position, there is an alternative. Punch three holes around the edge of the map and ask the students to pin it to the wall using one of the holes.

Attach a piece of string to the pin, with a weight tied to the end to pull it straight. If the pupils draw along the string, and repeat the experiment suspending the map from the other two holes, they should find that the lines cross at one point - the centre of gravity. If they place their finger where the lines meet the map will balance perfectly. This will work with any flat, rigid object.

The reason for this is because the centre of gravity is the point at which gravity is considered to act in an object. When suspended from a single point the object will tend align itself so that the centre of gravity is directly below the point for suspension.

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

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