Asked by: Robin Chambers, Preston

Films are shot as a series of individual frames taken at around 24 per second. If the wagon wheel is rotating at precisely the right rate, then a spoke in one frame will have been taken up by another spoke in the next frame, creating the impression the wheel is stationary. If they’re slightly out of sync, however, the wheels will appear to move in slow motion.

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Robert is a science writer and visiting professor of science at Aston University.