General Question

kelly's avatar

What is the phenomenon that makes a spoked wheel when turning at a certain speed appear to rotate backward?

Asked by kelly (1908points) March 27th, 2007
noticed a car ad with car moving down road, it had spoked wheels, as speed changed the wheel appeared to stop motion and then the wheel appeared to spin backward. It's an optical illusion, but what is the physics of it?
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9 Answers

bpeoples's avatar
Don't remember what it's actually called, but basically-- the camera is capturing at, say, 30 frames per second, and at its simplest, the wheel is spinning at 30 times per second. With an 18" wheel, that's something like 96 miles per hour, so it's more likely the wheel is rotationally symmetric. If the wheel has 6 spokes, than you could get the "stopping" at as low as 16 mph.
bpeoples's avatar
Same thing can happen with fluorescent lights (and possibly sodium vapor lights) -- so you might see the effect driving down the road.
bpeoples's avatar
So what's happening is that each time the camera takes a frame, the wheel is apparently in the same position (it's spun 1/6th of a turn, so the spokes are again in the same place). If you take a fast enough picture that the motion blur is minimized, then it can look like the wheel is totally still, although the car is continuing to move.
bpeoples's avatar
If the wheel is spinning slightly faster, then you get the spoke just forward of the last position, and it looks like the wheel is spinning slowly. If the wheel is spinning slightly slower than the frame rate, then the spoke will appear just behind the previous position, and the wheels will look like they're going backwards.
Perchik's avatar
Cool! Good to know
andrew's avatar
Can't you see this with the naked eye, though?
sarahsugs's avatar
Yes, I have seen this often, not on TV but in reality, that a car wheel appears to be spinning slowly backward. A physics teacher once told me that some scientists take this phenomenon as evidence that the brain processes visual information the same way a movie camera does - in isolated "frames" seen very quickly and very close together. I have no further evidence to back this up, but it is an interesting theory.
segdeha's avatar

I think it's magic.

Shuttle128's avatar

I believe this phenomenon is similar to that of aliasing in the digital capture of an analog signal. As the signal frequency changes with respect to the sampling frequency, you can get the illusion that the signal is moving in the opposite direction.

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