Naked Science Forum

Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: george on 04/11/2002 21:44:03

Title: Wagon Wheels
Post by: george on 04/11/2002 21:44:03: Whilst watching a crappy old "western" the other day my wife commented that the wheels on the carts go backwards. You never see this in 'real life' so it must be a trick of the camera presumably - but why does it happen ? Anyone know ?

Georgey[:o)]
Title: Re: Wagon Wheels
Post by: pat on 08/11/2002 01:31:11: I think it might be something to do with strobe effects ? But I might be wrong...

Heeelp someone
Title: Re: Wagon Wheels
Post by: chris on 24/04/2003 18:04:32: This is definitely one for Quantumcat to answer !

Chris
Title: Re: Wagon Wheels
Post by: Quantumcat on 25/04/2003 08:26:43: I know!!!!

Because:

you know how movies are made by taking one picture at a time? The frames the cameras take are timed so that, if the speed of the wheel is right, it takes a picture, the wheel spins around, and the camera takes another picture when the wheel is a little past the point it was on the other picture. So you see:
The wheel.
The wheel slightly round the wrong way
The wheel even more round the wrong way
etc. etc. etc.
So the wheel looks like it's going backwards!!!
This happens more in "old" films because the cameras were crappier and took pictures more slowly, so the effect happened at slower speeds. You can see the effect nowadays on car wheels, usually.
Title: Re: Wagon Wheels
Post by: pat on 25/04/2003 10:29:44: I too thought that it was down to a strobe effect on old cameras until the other day when I witnessed the same 'illusion' happening to a car's wheels - in broad daylight. No camera involved. How do you explain that ?

Pat
Title: Re: Wagon Wheels
Post by: Quantumcat on 26/04/2003 06:08:44: You have crappy eyes? (lol sorry)
Title: Re: Wagon Wheels
Post by: Pappy on 14/11/2003 20:07:41: Quantum......are you sure about that. I, too, have observed car wheels that seem to move backwards and I'm pretty sure my eye's are of non-crappy status.
Title: Re: Wagon Wheels
Post by: Quantumcat on 15/11/2003 08:44:35: I think the car wheels just move at the right speed so that the image stays on your retina until it moves around and your retina gets another image. I don't know the exact length of time an image stays on the retina, if someone tells us we can work out exactly how fast the vehicule was going that you saw this effect on (and if we know the circumference of the tires too of course)

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Title: Re: Wagon Wheels
Post by: bezoar on 15/11/2003 13:01:57: Could it have anything to do with the speed of light and the time it takes for the brain to process the image?

Bezoar