Science corner

3rd October 2003 at 01:00
Every object has a centre of gravity: Sophie Duncan gets the better of an uphill task

Can science help us to do magic? By understanding the centre of gravity we should be able to do the impossible and make something roll uphill!

This experiment was first performed in the late 17th century. It has been used to help people understand the principle of centre of gravity ever since. The photograph shows an example of this experiment on display at the Science Museum in London. It is part of the George III collection.

The experiment requires some preparation before the lesson. Create a device made up of two cones. It is easiest to make this from two plastic cones stuck together, but you can use thick paper. The cones need to be of equal size, and secured together so that the resulting object is thick in the middle and narrow at each side (as seen in the drawing). Now make a slope for the experiment. This is most easily made from two wooden sticks (metre rules work well).

Place the sticks on a table, putting the ends on a book, slightly higher than the width of the widest part of the joined cones. The sticks should not be parallel, but close together on the table and further apart on the books - like a triangle.

During the lesson ask your students what they expect will happen if you put the double cone at the bottom of the slope.

Explore their answers. Then demonstrate the experiment. When you place the double cone on the sticks, the object will roll uphill. Can they explain what happens?

The principle being explored is the centre of gravity of an object. An object will come to rest when the centre of gravity has reached its lowest point.

At the bottom of the slope, the sticks are close together and so the double cone sits in an elevated position. At the top of the slope, the body of the double cone sits in a lower position. The double cone therefore appears to roll uphill, although the centre of gravity moves downwards.

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

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