Why is it that when a car moves rather fast, the wheel seems to be moving in the opposite direction?

Question

I have noticed the same thing for ceiling fans. It always seem like that they are rotating in the opposite direction when they move really fast.

Answer 1

It's a strobe effect. If you only see an object with periodic flashes, and the object is going with a periodic motion, it can appear to stop or go backward.

Imagine a wheel turns once per second.
You flash it once per second--it looks still.
You flash it slightly more slowly--it rotates forward slowly.
You flash it slightly more quickly--it rotates backwards slowly.

Now really, the wheel and the flashes are much faster, but that's how it works.

It's fun to do tricks with strobes like spinning your hands really fast. By adjusting the speed, you can make yourself go backward or stop.

Any motion picture acts like a strobe, because it takes frames periodically.

Answer 2

It's an optical illusion caused by sampling rates, that is, how often the position of the spinning object is observed. Even though human vision in fairly continuous, the blinking of the eye introduces a sampling rate at certain times. However, this phenomenon is much more evident when the motion is viewed on a recording, where the sampling rate of most film and video is 24 frames per second.

In order for observations of a rotational body to be accurate, the sampling rate must be at least twice the frequency of the object. For the case of a wheel with, say, 20 spokes, the frequency is not the number of full turns per second, but rather the number of spokes passing a given point each second, because we can't tell the spokes apart. If the sampling rate is too slow, less than twice the frequency of the object being observed, then more than half a cycle passes between observations, and the observed motion is not an accurate representation of the actual motion.

For example, if the wheel with 20 spokes turns five times per second, that's a frequency of 100 spokes per second, or a period of 0.01 s. If the observations also had a frequency of 100 Hz, the spokes would appear to never move. If the sampling frequency were 133 Hz, the spoke would move 3/4 of the way to the position of the next spoke, but it would appear to have moved 1/4 of a space backwards instead; the wheel would appear to be spinning backwards. If the sampling frequency were 200 Hz, the spokes would appear to move halfway to the next or previous one, and then either the rest of the way or back to the original position; it would be unclear. With a sampling frequency of 300 Hz, the spoke clearly moves 1/3 of the way to the next one, then an additional third, and then completes the motion; the movement is clearly seen in the correct forwards direction.

Answer 3

Without getting mathematical, it's because our brains process the visual information at a certain rate. When we view the spinning object at a certain fast rate above our processing speed, the information that the eye sends to the brain, and the rate at which it is processed is out of sync, so the info obtained and the info processed makes it seem like the wheel is going backwards.

It's like seeing lines on a TV screen. When the rate of the scan on the screen is too fast and out of sync with our processing, we see lines moving backwards.

Answer 4

Because of the strobe effect of the light source.
In the movies because the moving images are a succession of still images that time to time are synchronized or dis- synchronized with the position of the wheels

Answer 5

it is how the light shines on it on that particular moment

also if you put a strobe light next to dripping water it seems that the water start to drip up

Answer 6

you HAVE to check out this site- it will explain it MUCH better than I could.

http://www.michaelbach.de/ot/mot_strob/index.html