Attractive model

29th December 2000 at 00:00
A box, a ruler and an elastic band are all that it takes to explain gravity, says Anne Goldsworthy

I just don't get what this gravity thing is all about," said a key stage 2 pupil, and the looks on the faces of a few of her classmates showed that many of them were of the same opinion. We needed another way of thinking about it which would help them make a link between gravity and something else they already knew about.

I chose the of analogy the "gravity box" (Figure 1). The box represented half the Earth. The pupils imagined it as a bit like an invisible elastic band between all objects and the centre of the Earth, rather like the elastic band between the ruler and the bottom of the box. Then a pupil held the ruler away from the bottom of the box and we talked about the forces on the ruler (Figure 2). The class recognised that the elastic band (gravity) was pulling the ruler down and that the hand was pulling it up and that when the ruler was still, these forces were balanced. When a pupil let go of the ruler the class had little difficulty in realising that the ruler was being pulled back to the top of the box by the elastic band (gravity).

After that, when I held up another object and dropped it, the children talked about it being pulled towards the Earth by gravity rather than just saying that t fell. Finally, I turned the box upside down, pulled the ruler away from the box again and let it go (Figure 3). Once again the ruler was pulled back towards the centre of the Earth and the children found it easier to believe that, even in Australia, things are pulled towards the centre of the Earth by gravity.

As with all analogies, this one has its drawbacks. The main one is that the further apart you pull two objects attached to an elastic band, the greater the pull between them. But with gravity the further apart two objects get the weaker the pull between them. It is a good idea to ask children to identify any problems with analogies you use so that they realise analogies are just tools to help us picture something and are not perfect representations of what is happening.

Best of all is to ask children to make up their own analogies. One pupil was asked to think about why he saw the flash of a firework before he heard the bang. Having given it some thought, he said: "Well - the light and the sound set off from the firework at the same time but the light's like Linford Christie and the sound's like a snail and that's why the light gets to me first."

Not bad!

Anne Goldsworthy is an independent consultant and past chair of the Association for Science Education primary committee.

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