How to balance a seesaw

6th May 2005 at 01:00
Sophie Duncan weighs up how to balance a seesaw

Everyone loves playing on a seesaw - but what can seesaws teach us about science? This week's science corner explores science in the playground.

Discuss with your students how a seesaw works. Ask them to predict what would happen if two children sat on one end of the seesaw and one child sat on the other end. How could the children balance the seesaw?

Get each group of students to make their own seesaw to experiment with.

They can be made from a ruler and an angled block (triangular in cross section). To stop the ruler slipping, wrap two elastic bands either side of where the ruler touches the top of the block.

Secure the block in place. Balance the ruler by placing equal masses (for example, two coins) on either side of the pivot. Note the distances the coins are from the pivot. Repeat using two coins on one side of the pivot and one coin on the other. Do your students notice any relationship between mass and distance?

Encourage them to try with more coins. They may notice that if they double the mass on one side of the seesaw, it needs to be half the distance from the pivot.

Go into the playground and ask two children to volunteer to sit on the seesaw, one on each end. Ask pupils to work out how to make the seesaw balance. Once it is balanced, ask another student to measure the distance between each child and the centre of the seesaw (the pivot) and note it.

Ask a third child to take part in the experiment by sitting with one of the children already on the seesaw. Can the students work out how to balance the seesaw? Test their theory and take down the measurements again.

Seesaws are governed by a simple equation stating that to achieve balance, the force exerted on one end of the seesaw times the distance between the force and the pivot should equal the force exerted on the other side of the seesaw times the distance of that force from the pivot. The force is the mass of the object multiplied by g, the gravitational constant.

Therefore, if the mass on one side of the seesaw is larger than the mass on the other, its distance from the pivot needs to be shorter in order to balance the seesaw.

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