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For many years i have been wondering about this. When a car stars moving the wheels seem to work normally but as the car speeds up, the wheels seem to roll backwards. Of course that does not happen in all cars, so i am guessing it depends from the type of the rims, right?

So, why do car wheels seem to roll backwards after some specific speed?

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    $\begingroup$ Its an optical illusion : Wagon-wheel effect $\endgroup$
    – user49111
    Nov 23, 2014 at 14:37
  • $\begingroup$ do you mean in a film or on television - or when you are driving and you see a car on the road? $\endgroup$
    – tom
    Nov 23, 2014 at 14:49
  • $\begingroup$ Now that i think about it, i have only observed it in videos. $\endgroup$
    – George Smyridis
    Nov 23, 2014 at 15:54
  • $\begingroup$ I've seen it in real life, but only under certain streetlights. $\endgroup$
    – Mooing Duck
    Nov 24, 2014 at 18:10

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Its a stroboscopic effect. Your eyes can only process information properly which is below a certain amount of frames per second. When wheels rotate slowly we see them in the direction which they are supposed to be rolling in, but when the rotation of the wheel crosses a certain speed, we observe its motion in a totally different way. Assume we observe a point on the wheel. At slow speeds, we'd watch it in this order: 30, 60, 90, 120 (in degrees) and so on. but when that certain limit gets crossed, at one second we may observe the position of the point to be at 30 degrees, but in the next "frame", the wheel might have rotated an almost complete cycle and we may see the point at 25 degrees! If enough of these events happen in a sequence, our eyes observe them as the wheel rotating backwards. I hope I've been clear enough!

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    $\begingroup$ This answer seems to imply that the strobe effect is caused by some frame effect in the vision process itself, when we directly observe something rotating. Am I misreading? $\endgroup$
    – DJohnM
    Nov 23, 2014 at 19:00
  • $\begingroup$ why not edit your answer to put the link in that The Pragmatick mentions in comment to the question above. $\endgroup$
    – tom
    Nov 23, 2014 at 19:31

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