system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: At certain speeds, why do wheels (on cars, bikes, etc.) look like they're going backwards?

EDIT: Thanks for all the replies!
gpt
Say you have a film camera running at 30 frames per second.  Point it at a wheel with a big bright pink line on it.  Then start turning the wheel slowly (let's say 1 rotation per second).  Every time the wheel turns slightly, that bright pink line will move slightly more forwards around the wheel.  In each successive frame you can see that pink line moving slightly more and more forward.

Now, increase the speed of the wheel (to say 5 rotations per second).  You will see that pink line moving even more and more quickly around the wheel.  Everything up this point makes sense, right?

Now let's say you make that wheel rotate so fast it's rotating at 30 rotations per second.  If you film this with that camera and play it back, the wheel looks like it's not moving!  Why?  Because every time the wheel completes a rotation, the camera captures that pink line in exactly the same place as it was before!  The wheel will look like it's standing still!

Decrease the speed of the wheel to 29 rotations per second.  If you film this with that camera and play it back, the wheel looks like it's going backwards!  This is because just before the wheel completes a full rotation the camera will take a picture.  Then just before it can complete the next rotation, the camera will take another picture.  Every time the camera takes a picture the wheel completes most of a rotation, but not an entire one.  The wheel then looks like it's turning backwards!