Does anyone know...

1. Member
Join Date
Dec 1969
Posts
95

## Does anyone know...

what physics law, or theory, is used to explain the appearance of a car wheel spinning "backwards" when its going forward?

2. Senior Member
Join Date
Dec 1969
Posts
16,931

## I don't think...

...there is a law or theory really.<BR><BR>The physics is probably quite easily explained, though.<BR><BR>How do you know whether something is rotating? You fix your eyes on a noticable spot and watch it go around or stay still. Or, if there are no very noticable sections, you see how the light bounces off, and you theorise that it must be spinning to produce those effects...<BR><BR>However, the eye can only take a certain number of "stills" per second - around 60, I think. So, think about it. If your eye takes 60 stills per second, but the car wheel spins at 60 rps (it makes 60 rotations per second), your eye takes a "picture" of the wheel when it&#039;s at exactly the same rotation every time round - to your eye, it&#039;s stationary.<BR><BR>Working with that physics, lets say it is spinning at 50rps. That means that every second, it&#039;s ALMOST completed a rotation, but it&#039;s nearer the end point than the start, so your brain says "it&#039;s gone backwards", rather than "it&#039;s gone forwards"...<BR><BR>Anyway, that&#039;s what I&#039;m guessing...<BR><BR>Craig.

3. Senior Member
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Dec 1969
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7,686

## correct <eop>

.

4. God
Senior Member
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Dec 1969
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18,177

## That's pretty sick.

Hey Craig, I just installed Trillian again.. can you e-mail our my ICQ #s?

5. Senior Member
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Dec 1969
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11,334

## Alcohol <nm>

boo

6. Senior Member
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Dec 1969
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2,930

## bravo

i could have said it better but good stuff :-P

7. Senior Member
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## Intresting "title" ReaperMan

what else dont you do.....tell us something we did not already know :p<BR><BR>

8. Senior Member
Join Date
Dec 1969
Posts
1,913

## But could you do better than this?

It is a "stobascopic" effect, and is often seen under discontinuous lighting conditions (e.g. fluorescent lighting, tv, movies). The explanation for this is simple and has already been covered.<BR><BR>For its manifestation under continuous lighting conditions (e.g. sunlight, incandescent lighting), it turns out that there is, in fact a sampling rate in the optic nerve, under certain conditions. Time-dependant visual stimulous is sent down the optic nerve at particular frequencies. "time-dependent visual stimulus is encoded by the collective activity of many retinal ganglion cells" Small ganglia synchronize their transmissions down the optic nerve perhaps to help in compression of information - so it seems that for at least time-dependant visual signals (i.e. a regularly rotating wheel), there is a sampling rate of about 30Hz down the optic nerve. <BR><BR>See these for more:<BR><BR>http://rhino.harvard.edu/Publications/Meister_1996_Multineuronal.pdf<BR>http://rhino.harvard.edu/Publications/Crevier_1998_PeriodDoubling.pdf<BR><BR><BR>

9. Senior Member
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Dec 1969
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2,930

## took the words

right out of my mouth.

10. Member
Join Date
Dec 1969
Posts
95

## Good gravy

I am of small brain and cannot handle this right this second... but thanks for the links - reason I ask is twofold:<BR><BR>a) I was on the road and noticed it and its one of those things a kid would ask (now I know to tell him its a stobascopic effect, now shut up and sit down)<BR><BR>b) Good driving games (Gran Turismo, GTA), simulate this effect programmatically. I&#039;d be curious to know how they do so.

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