Eli5: Why do wheels look like they’re spinning the wrong way when going fast?
In: Physics
You can think of like this:
Every morning your neighbor walks their dog. You can see them from the window. They take exactly 10 minutes to walk path visible from your window, from appearing from one side to disappearing on other side.
Imagine that every morning you glance out of the window for a second and take a photo of them.
First morning you caught neighbor late, when they had almost passed your window. Let’s say, on 9th minute of their walk. You wanted to see them again, so next morning, you woke up a minute earlier and photographed them when they were about 80% done with the path behind your window.
Next morning, you glance out of the window another minute earlier, and even earlier next day, and so on.
Now you’re looking through the photos in order of taking them. What do you see on photos? Your neighbor and their dog going backwards.
Brain does same thing if the speed of moving thing is just about right. It’s “taking photos” in a way that motion appears backwards, even though it’s not. Because just like your camera or phone, it’s realizing moments of something’s position, like snapshots. Then tries to put them together. And your brain is just being logical – it was there and now it’s there, probably moved backwards. Same as with the photo example.
This is an optical illusion caused by closely matching the speed of the wheel to the perceiving rate of the observer, and not really because it’s going fast.
Imagine you put a piece of yellow tape on top of the wheel to visualize the rotation, and then spin the wheel 10 rounds per second.
If you then have a camera that starts taking pictures when the tape is at the top, and then takes 10 pictures per second, each picture will be taken right when the tape is on top, and the wheel will have the same rotation in each picture.
The result is that to anyone looking at the video recorded by the camera, the wheel appears to be stationary, because each picture is of the wheel in the same rotation.
Now imagine we instead take a few more pictures per second than the wheel is rotating, for example, 11 frames per second.
Again we start recording when the yellow tape is at the top. Because we have more frames per second, each one will be slightly closer together in time than with 10; that is, the camera is taking pictures faster than the wheel spins.
So the second picture will be taken slightly before the yellow tape has reached the top again. And this goes for each new image we take. Each time, the tape will not quite have reached where it was in the previous picture.
So now when we look at the video, each image will be of the wheel slightly less rotated forward than the last, and thus, to someone watching the video, the wheel will appear to be moving very slowly backwards.
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