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A security camera, taking pictures each half a second, films a cyclist going by. In the film, the cyclist appears to go forward while the wheels appear to go backwards. Why?

Speeding Up, Slowing Down

Age 11 to 14 Challenge Level:

Take a look at the interactivity below which shows regular polygons "rolling" along the horizontal surface. It leaves a trace of the path of the red dot and on the graph it records the speed of the travelling dot.

Experiment by positioning the red dot at the centre of the polygons, at one of the vertices or at the centre of one of the sides of the polygons and explore how this affects the speed / time graph.

Full Screen Version


Challenge:

Can you now work out what produced the following speed / time graph?
The rolling polygon had a radius of 30.
Can you work out how many sides it had and where the red dot was placed?
Try to explain how you worked it out.

Speed / Time graph