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JustJessica's avatar

Why do wheels always look like they are moving the opposite way when watching TV?

Asked by JustJessica (4054points) March 23rd, 2011

I may be wrong, but I’ve noticed that once a car gets up to speed while being filmed the wheels always look like they are going backwards. Can you explain this optical illusion for me?

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4 Answers

wundayatta's avatar

This is how wikianswers explains it:

Wheels appear to rotate backwards in movies because of what is called, ‘strobe effect’. When filming, the camera actually takes a series of pictures. An example would be:
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 continuous motion of the wheel, but rather the positions of the wheel every 1/50th of a second. You can see the same effect when looking at a fan turning in a room with Fluorescent lighting. The bulbs flicker at a certain cycle thus causing the illusion.

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YoBob's avatar

Great question!

The reason is that unlike the real world, motion pictures are composed of a series of still pictures that are flashed before our eyes at a speed that is fast enough that our brain puts it together as smooth motion (usually somewhere between 25 and 30 FPS). In the case of spinning wheels the illusion is caused by the fact that the wheel does not make a full revolution between frames so with each discrete image the image of the wheel appears in a position that is a little bit behind what it was in the previous image. String those images together at motion picture speed and it looks like the wheel is moving backward.

WasCy's avatar

The previous respondents have come close to explaining the answer, but it’s not that the wheel is turning less than 50 rpm.

For example, assume a vehicle on 2’ diameter tires traveling at a speed of around 25 mph.

The circumference of the tire is 2 * pi * radius 1 = approximately 6.3 feet

The vehicle traveling 25 mph is traveling 2200 feet per minute. That translates to approximately 350 RPM for the tire, or almost 6 revolutions per second. If the camera shoots at 50 frames per second, then over the course of, say, 10 seconds you have 500 frames of film showing the tire rotating about 60 times.

What becomes obvious is that the rotating wheel will almost never synchronize with the film speed, so that the wheel will appear to be “advancing” with each successive frame. Depending on the synchronicity of the wheel and camera, it very well might seem that the wheel “advances” in the film (and you’d never notice an oddity if it did, because you expect that to be the case), but when the wheel’s rotation appears to lag because of the filming, and each new “snapshot” appears to be a little “less forward rotated” than it was for the last snapshot, then your brain interprets the wheel as rotating in reverse, and you certainly notice the discrepancy, when the vehicle moves forward.

JustJessica's avatar

Woo Hoo thank you guys for the great answers!!! It makes total sense now!!!

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