wheels

[vauxhallmush]

how fast have you got to travelling before your wheels look like they are going backwards i.e like on car programs such as top gear .

[Dr Wu]

It's nothing to do with speed really.

It's basically a strobe effect caused by the fact that your TV picture is actually constantly being refreshed so the wheels look like they are going backwards.

It's why computer screens look funny on TV too.

So you only see this on TV and not for example if you were actually at a racetrack

[rojash]

A TV picture is 25 frames per second.
Imagine a disk with a single blank mark at the 12 o'clock position, for the first frame.
If , when the second frame is taken the mark has moved to anywhere up to 6 o'clock, the wheel will appear to travel forwards.
Speed up the wheel until the mark passes the 6 o'clock point before the second frame, and the wheel will appear to go backwards.
Continue increasing the speed until the wheel makes one complete revolution in a 25th of a second and it will appear to be stationary.
Now carry on increasing the speed. so that the mark makes it all the way round and back to the 1 o'clock position. The wheel will once more appear to be moving forward, but much more slowly than it really is.
As you continue increasing the speed the wheel will appear to go forwards then backwards then forwards again until it's moving too fast for you to distinguish the mark.

[chadad]

Just as a matter of interest. If you know of a stretch of road that is bordered by a fence (The upright slats type).
Watch through the fence as a car passes on the other side.
You get the same illusion as on the movies.
The wheels appear to be revolving backwards.
chadad.