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## 'Illusion' printed from http://nrich.maths.org/

A security camera, which takes one picture each half a second,
films a cyclist going by at 8.5 miles per hour on a bike which has
wheels of diameter 24 inches. There is a reflector fixed to the
spokes of each wheel of the bike. Explain why it is that, in the
film, the cyclist appears to go forward while the wheels appear to
go backwards. If the cyclist goes a little faster the wheels appear
to be stationary. At what speed does this happen?

You will need to know that 1 mile = 1760 yards, 1 yard = 3 feet
and 1 foot = 12 inches to do this question. Can you also change the
numbers (not necessarily by direct conversion) to pose a similar
problem in metric units?