When tyres move fast, it seems like they are moving in the oppsite direction. Why?

Question

I don't want to know what it would look if captured on film. I want the answers w.r.t human eyes, viewing normal wheels (no fans) and the apparent reversed direction of wheels.

Answer 1

This only ever occurs when something is capturing images non-continuously.

See the answer to your previous question.

Answer 2

OK. talking non video, pictures, or fans... Uhmm.

If you take a disk and spin it in one direction,with point A pointing north, then point A moves oround throught various positions untill it is back where it started.

Now you can only see the positin of A say every 0.00002 seconds. Now if the position of A has moved so far around, that it is closer to north from the oposite side than the spinning direction within 0.00002 seconds, it will appear as if it moved backwards.

In the case of tires, the groove on the tire is repeated almost 200 times around the tire. This means that the tire does not have to go around more than half a turn within 0.00002 seconds, but only one 200th of a turn. So when the tire turns at such a speed, that the next groove is closer to the previous groove than that groove's position, you will register it as that groove moving backwards. You are infact looking at the next groove that has moved forward, but the brain analyses it as it seems fit.

OK reading that makes almost no sense. Read it a couple of times slower. Man, I wish I could draw pictures on this answer.
Therefore it lloks like it isgoing backwards.

Answer 3

Assuming you're talking about watching spinning wheels on TV or in a movie.

This is a result of the particular frame rate that the video or movie image is being captured at.

At a certain speed, the spokes of the spinning wheel become 'out of sync' with the frame rate of the video capture device.

For example, if your video frame capture rate is 30 frames per second, and the wheel is spinning at exactly 30 revolutions per second (or some multiple of 30 rps) the wheel will look like it's not spinning at all.

If you vary the wheel's speed just a little +/-, it may appear to spin slowly forward or backward. It's kind of an optical illusion based on the sampling rate (video frame rate) and the speed of the spinning wheel.

Answer 4

dbucciar is correct. If the wheel rotates at the same rate as the camera takes pictures, the wheel appears stationary. If it rotates 24 times per second, and your video camera captures 25 frames per second, it will not have rotated as far by the next time it is captured, and so will appear to have moved backwards a little.

In addition, our eyes also process data at a given rate, so you can see the same effect when looking with your eyes.

Fluorescent tubes strobe at the mains rate (50Hz) and so can have the potential to add to the effect. Circular saws can appear stationary under such lighting!

Cheers,
Ben

Answer 5

In addition to what some of the other answerers have said, some cars have a fan on their tires which spin at a different rate, especially when the car is slowing down. This may give the illusion that elements on the fan are spinning in the wrong direction (relative to the tire).

Answer 6

well, they are! according to Newton's laws of physics, to every action is an equal and opposite action. your car is moving forward, so your tyres are moving backwards! q.e.d.