Why does a tyre seem to spin backwards when really it is going forward?

Answer 1

It's called optical illusion.....

Answer 2

If you are seeing a film, then, yes, part of it has to do with the speed of the film versus the rotational frequency of the tire.

However, if you see a tire or fan accelerate, you will notice the same thing happening for a small period of time. This has to do with the frequency of light itself (and how ours brains perceive it) and not of a film.

The film scenario is easier to explain. Let's say you have a fan blade with a single colored dot on it. Assume the fan is rotating at 25 rotations per second (rps). If you film the object at 25 frames per second (fps), the dot will not appear to move at all because the dot will be in the same location in the rotation each time the frame is captured.

If you film it at 20 fps, the dot will appear to advance each frame because the fan blade will have rotated more than 1 time each time it is filmed. For example, if the dot starts at 12:00, it will have made 1 revolution plus some before the next frame is shot. So, on the next frame it may be at 2:00. Another frame goes by, the fan will have made one complete revolution plus some, so it appears to be at 4:00, and so on.

But, if the film speed is faster than the rotational speed, the dot will NOT have made one complete revolution each time a frame is shot. So, if the dot starts at 12:00, it may have only made it to 10:00 by time the next frame is shot. In that case, it will appear to be moving backwards.

Like I said, the same is happening in the non-film case. Instead of a film speed, it has to do with the frequency of the light (either from a bulb or from the sun) and how we perceive it.

Hope this helps.

Answer 3

This has to do with the frame speed of the camera being used.

Answer 4

This is known as the stroboscopic effect.