Why can wheels appear to be turning backwards at certain speeds?

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Offline thedoc

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Lorne Henry  asked the Naked Scientists:
   Dear Chris,    Why do wheels sometimes appear to be going backwards and what speed do they need to be going for this effect? Also; If seated in a stationary train next to another trainfacing in the opposite direction, when one starts to move why can it bedifficult to know for a moment which one is moving?  I'm assuming it's some trick of the eyes.    Lorne Henry, countryside NSW Australia.

What do you think?
« Last Edit: 12/10/2016 18:53:02 by _system »


Offline chris

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Hello Henry

In the case of the car wheels, this is a stroboscopic effect most usually seen at night on an illuminated road. This is because the mains electricity powering the street lights is alternating current with a frequency of about 50Hz. This means that the light is flashing on and off 100 times per second, showing you 100 glimpses per second of the moving car wheel. At a certain speed of rotation it will look like the wheel is staying still, because it will be turning at precisely the same rate that the light is flashing on and off. But at changing speeds it will complete different proportions of a complete revolution between glimpses, giving the impression that the wheel is turning slowly. At the "right" range of speeds it will look like the wheel is going backwards.

The observation of the train moving is to do with how your visual system processes information. Because the visual scene of the other train moving away from you dominates your vision, the brain concludes that you must be moving rather than the other train. This is because the brain is more used to seeing the world remain stationary and you moving relative to it, so you are temporarily tricked.

I never forget a face, but in your case I'll make an exception - Groucho Marx