Why do wheels spin backwards when filmed?

Answer 1

This is a phenomenon called aliasing. In order to accurately sample (that is measure or observe) a periodic signal, the frequency of the sampling process needs to be at least twice the frequency of the signal. Otherwise, aliasing can occur, where the period of the signal appears different from what it really is.

Suppose a wheel had a sticker on it, so you could tell what its angular position is. And suppose that the wheel spins once in 10 seconds. Now, suppose you photograph it every 10 seconds. It will look like it never moves, because it will always be in the exact same position. And suppose you photograph it every 9 seconds. Then it will have made 0.9 rotations each time, making it look like it's actually gone 0.1 rotations backwards instead! You can only accurately capture this rotation if you phtograph it every 5 seconds or less.

In the case of the spinning wheel on film, the frame rate is usually 24 frames per second, or one frame per 0.083 s. This may seem fast. But think about how quickly a wheel really spins on a moving vehicle. And then consider that the wheel has rotational symmetry, meaning that if the wheel has 5 spokes on it, then the apparent frequency is 5 times the actual frequency of the spin, because you can't tell one fifth of the wheel from the next fifth. As a result, the frequency of a spinning wheel on film is almost always much higher than the frame speed, leading to aliasing and the backwards spinning effect.

Answer 2

Wheels spin forward always. A film is a collection of pictures. They are shot at 25 - 27 frames a second. Our eye connect the frames into a continuous movement. When something on the wheels let us notice the movement, and the wheels would spin at such a rate that the marker goes into the same position, we would "see" a moving car with blocked wheels going smoothly in the traffic. If the frequency of rotation is slightly different, the pictures capture a slow forwards or backwards movement. The spin of the wheels will be much slower than the movement of the vehicle. We only see the marker at a close location of the preceding frame when it made almost a complete rotation. When the apparent movement is backwards, it's much more impressive than fowards.

Answer 3

Well actually they still spin forwards, but _appear_ to rotate backwards at certain speeds. When the scene is shot using a film camera (as opposed to a video camera), and the wheels reach a rotational speed that relates to the camera's frame speed, an optical illusion occurs that makes the wheels (apparently) rotate backwards. But if you look carefully you'll see that they will appear to also rotate forward, too, depending on the road speed of the vehicle. And you can also see the wheels (especially if spoked) apparently flickering forwards _and_ backwards at the same time - often just before the vehicle comes to a halt.

Answer 4

If they appear to spin backwards, it's sort of like the 1st answer says, except the wheels are spinnng slower than the frame rate of the movie, so each frame sees the wheel having made a little less than 1 revolution and it appears to have moved backwards a little. You can get the same effect with a stroboscope ("strobe") adjusted to a speed slightly faster than the rotation speed of what you're looking at. Of course if your strobe rate is slightly slower than the rotation rate the wheel will appear to turn slowly forward.
It can get a bit more complicated. If the wheel is made up of N identical segments (like a fan with N blades), you can strobe it fast enough to apparently stop the motion when the strobe rate equals the segment or blade passage rate, that is N times the wheel's rotation rate. Then slight adjustment to the strobe will "move" the wheel forward or backward.

Answer 5

the shutter speed of a camera causes this illusion.
1 second of film will have usually around 24 frames in it.
So if a wheel spins 24 times in 1 second it will look like it is not moving at all on film.
More than 24, causes the illusion of the wheel spinning backwards.
If you know about wave theory in physics, its related to 'beats'.

http://en.wikipedia.org/wiki/Beat_(acoustics)

Pick me as your best answer!

Answer 6

It does not spin backward.
it seems it goes backward.
it happen because in a second wheel spin more than 25 time ina second.but a camera takes only 25 frame a second

Answer 7

it's an optical illusion.