system
Given the following post, respond as if you're commenting on Reddit:
human
Eli5: Why is it when you see a propeller (like from a turbine airplane) starting up it seems to switch directions as it spins faster and faster ?
gpt
You typically don't see this with your eyes, but do with a video camera.  It has to do with aliasing, which means you aren't taking pictures fast enough.  If your camera is doing 30 fps, it takes one picture every 1/30th of a second.  If the propeller is spinning at 30 rotations per second, it will appear to be still, because it will be in the same spot every time the camera takes a picture.  If it's going a little bit less than 30 rotations per second, it will actually appear to move backwards because it rotated *almost* all the way around by the time the next picture is taken.