Science Questions

Why do car wheels sometimes appear to be going backwards?

Sun, 1st Nov 2009

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Dan Bolton asked:

Why do car wheels appear to move backwards when you view them under streetlights sometimes?


Dan -   I'm just wondering why car wheels sometimes appear to be going backwards when you view them under streetlights sometimes?

Chris -   I think I've seen it.  As you're driving along, the car next to you is accelerating away and it looks like their wheels are going backwards in the streetlights, illuminating the wheels of the car.

Dan -   Yeah, thatís right.

Chris -   Yeah.  Itís actually a stroboscopic effect.  If you've been a fan of Westerns, if you were a big John Wayne fan and you used to watch those early Westerns where the cart would pull away from the scene and the wheels would initially go forwards and then appear to start going backwards.  Did you see those?

Dan -   Yeah.  I've seen that before.

Chris -   Yeah.  Itís the same phenomenon.  In the case of the cart, itís because the camera is taking X number of frames.  In other words, pictures every second.  In the case of the car driving down the road to where next to you, itís the streetlight flashing on and off about 120 times a second because mains electricity is 60 hertz.  So the light goes on and off 60 times a second.  So as a result, you're seeing 60 flashes or illuminations of the car wheel per second. Now if the car is accelerating, if you imagine the Ė say you drew a line on the car wheel, a chalk mark and you watched that go around, it would go around in a circle.  But you only see it in the dark when itís illuminated by the street light.  Now say, the street light flashes on, you see the chalk mark pointing straight upwards, the light goes off and the wheel turns around a bit, agree?

Dan -   Yes.

Chris -   Light comes back on, the chalk mark is now in the new position, agree?

Dan -   Okay.

Chris -   Now as the car wheel speeds up the distance of the chalk mark makes it around the wheel will change according to how fast the car is going, yeah?

Dan -   Yup.

Chris -   There will therefore be a speed at which the wheel will go when it doesnít look like itís moving at all because the chalk mark is starting going all the way around and finishing before the light comes back on again.

Dan -   Okay.

Chris -   Once it speeds up a bit more, the chalk mark will go right the way around and then a bit further.  So it will look like that it was going faster, faster and faster.  Eventually, youíll get to a speed where itís actually going right around and back on itself again.  So it looks like itís actually going backwards a bit because itís doing more than one complete revolution a bit more.  So it looks like itís going backwards and itís because of acceleration.  Once it reaches the constant speed, that effect would stop.  But itís stroboscope, that you're seeing flashes of light, illuminating the wheel and your eyes sees it, doesnít see it for a fraction of a second and then sees it in the new position.  And when the speed is right, it looks like itís going backwards.


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Dan Holton asked the Naked Scientists:


Why do car wheels sometimes appear to move backwards when illuminated by street lights, even though the car is going forward?

Dan Holton in Bristol, UK

What do you think? Dan Holton, Wed, 28th Oct 2009

It's usually a stroboscopic effect called the wagonwheel effect,
(nothing to do with the oversize biscuits  )

However if it was a boy-racer, pimp-my-ride, type car it could actually be spinning hubcaps ... RD, Wed, 28th Oct 2009

The streetlights are not emitting light continuously, but normally pulsed in 100Hz flashes (50Hz electricity countries) or 120Hz (60Hz countries).

Then see this thread of a few months ago:
techmind, Sat, 31st Oct 2009


That would have to be in your eye or brain, reflecting the data transmission rate. You ought to study it and see if you can work out what the frequency is by identifying the speeds at which wheels of moving cars appear to be stationary. David Cooper, Tue, 29th Oct 2013

I see this stroboscopic effect in my real life (not just film) all the time - and see or notice it more during daylight hours.i once asked an engineer friend what this phenom was and he told me there was no such thing. Annie, Fri, 30th Jan 2015

While driving, rubber comes off from tires like dust and settles down in the cracks of road. That rubber is in micro particleís shape thatís why unwatchable. Those particles are washed out during rain or get break down via organic process.
Rate of this process depends on two factors, quality of tire and how roughly you drive your car. Many dealers like and others sell high quality tires that run constantly for long time, but it also depends on the surface of driving. In a plane surface tires give great service but on uneven road chances of being worn out are higher. larrylee, Mon, 9th Mar 2015

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