It is a familiar observation that spoked wheels do not always appear to be rotating at the

Question:

It is a familiar observation that spoked wheels do not always appear to be rotating at the correct speed when seen on films. Show that if a wheel has s spokes and is rotating at n revolutions per second, and the camera operates at f frames per second, then the image of the wheel appears to rotate at N revolutions per second, where

N = [FRACIT (-1)-1] f 2 2