Why do car wheels sometimes appear to be going backwards?

01 November 2009

Question

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

Answer

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, that's right. Chris - Yeah. It's 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. It's the same phenomenon. In the case of the cart, it's 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, it's 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 - 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 it's 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 doesn't look like it's 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, you'll get to a speed where it's actually going right around and back on itself again. So it looks like it's actually going backwards a bit because it's doing more than one complete revolution a bit more. So it looks like it's going backwards and it's because of acceleration. Once it reaches the constant speed, that effect would stop. But it's stroboscope, that you're seeing flashes of light, illuminating the wheel and your eyes sees it, doesn't 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 it's going backwards.

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