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Why do the wheels of a moving car appear to rotate backwards sometimes?

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2007-08-13 23:37:04
2007-08-13 23:37:04

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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Unless a brake is engaged, the wheels on a car in neutral can rotate backwards. If the car is in gear, wheels attached to the power train probably will only rotate backwards if the car is in reverse gear. If it is in forward gear or "park" they should not rotate backwards. Wheels not connected to the power train should be able to rotate either direction.

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it just looks it its not really going backwards, that's what they call an optical illusion.

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Uranus and Venus both rotate 'backwards', spinning clockwise when veiwed from above.

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Well, if you think about it, one is a consequence of the other. If you lie down on the ground and rotate (roll) west to east, everything around you that isn't moving will appear to you to rotate east to west (left to right if your head is pointing north).


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