Question

In movies why do wheels of a moving car appear to rotate backwards to the actual direction?

Asked by: Gary Blackwell

 Answer

Wheels appear to rotate backwards in movies because of 'strobe effect'. When filming, the camera actually takes a series of pictures. An example of how this works is as follows:

Imagine you have a spoked wheel rotating at 50 revolutions per second. If the camera is filming at a rate of 50 frames per second (taking 50 individual pictures each second) then the wheel would be in the same position every time the picture was taken. When viewing the film it would appear that the wheel was not moving. Now, slow the wheel down a little bit and it would be slightly behind the position it was in when the previous frame was shot. Now the wheel would appear as if it is rotating backwards because you are not viewing the continuos motion of the wheel, but rather the positions of the wheel every 1/50th of a second.

To read more about this effect check out: Rotation of a wheel by Philip Plait, Astronomer/Programmer (this reference was suggested to us by Christina Ratzinger.

Answered by: Doug Durdan (and Christina Ratzinger)

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