Watch the film below.
Full Screen Version

why not install a flash player in your browser so you can use this interactiviy?

Try to describe what happens in the film to someone else.

Imagine the dot starts at the point (1,0), turns through 20∘ and then stops:
Dot at 20 degrees

If the point now carries on, through how many degrees must it turn to finish the same height above or below the horizontal axis as it was when it had gone through 20∘ ?

Can you find a general rule for explaining when the dot will be this same distance from the horizontal axis?


You might like to try Round and Round and Round after this problem.

This film is based on one developed by Alf Coles - thank you to Alf for allowing us to use these ideas. You can watch all Alf's films at http://www.mathsfilms.co.uk