(eli5)Why does something spinning really fast appear to slow down or stop moving



You could imagine a car wheel, when a car is going really fast, it almost seems to stop moving, slow down, or even spin the opposite way. Why is this?

In: Physics

That happens because of your frame rate or the frame rate of movies. Lets say you track the movement of a hand on the clock (just one). Now your frame rate is something like 24 frames per second. Meaning 24 times per second you update the mental image of what’s in front of your eyes. And due to the fact that the fastest hand on a clock moves at 1 small movement per second, so you see 24 pictures for 1 movement.

Now suppose it wouldn’t move 1/60 of a circle at a time but would move straight from 12 to 6 every second. So 30x faster. Now from that movement from 12 to 6 you’d have 24 pictures of an interval of 30 seconds intervals. Meaning you’d already have a problem to see it passing the individual seconds marks. Now assume it does a full rotation in just 1 second. So again 24 pictures while it moves over 60 places so you’d only roughly see it every 2nd seconds interval on the face.

And now assume it moves 24 times faster than that. So 24 revolutions per second. And you’d have 24 pictures of that movement. The ironic thing is you wouldn’t see it moving at all because every time you’d take the picture it would have just arrived at it’s origin (again!). So you’d miss the entirety of it’s movement and therefore assume it stays still despite it moving really fast.

Now assume it moves a little faster than 24 revolutions per second. Now you would see it moving but it appears creeping forward very very slowly because you again miss the revolution entirely and just see how much it moved beyond the full revolution.

And so on if it moves faster than that you’ll see a fast movement again, but probably think of it as slower than it actually is because you don’t realize all the revolutions that were too fast for you to recognize. And every time the revolutions per second hit a multiple of the frame rate, it appears to stand still.

The reason your eyes see a series of still images in rapid succession as moving is because your eyes have a refresh rate, effectively.

Your brain basically remembers the signals coming from your eyes every fraction of a fraction of a second, and processes them into images. Then it processes new input from your eyes and processes them into images.