Chopping helicopters & now... reversing wheels?

[Richard]

I've not only wondered about the chop chop of a chopper, but I've also wondered why some items stop & turn in reverse even though they are rolling forward /forums/images/graemlins/shocked.gif.

I presume most/all have seen this from time to time.

Take a car wheel for example... driving down the road, you will see the wheel turn, as it goes faster, it's image blurs and a slower, blurred image of the "spokes" might be visibie...then apparently stops... and then it turns in reverse.

Why?


My theory..

Light has different frequencies. When witnessing this, the light acts much like a strobe light and as the wheel speed hits a certain speed, it catches up to the light frequency (slowing down of the visual) stops (matching the frequency/strobe) and then turns in reverse (surpassing the stroboscopic effect)


??

any merit?

If not, what causes it?

Yes..I admit...this is one of those things I wonder about every time I see it /forums/images/graemlins/confused.gif /forums/images/graemlins/blush.gif /forums/images/graemlins/tongue.gif

 

[Gatorboy]

Light -- artificial not sun -- will cause you to see this. This is the same principal that a turntable (not many around nowadays) uses. You could adjust the speed until the reference image stood still -- then you knew you were at 33 1/3.

Also, filming it will do the same thing (i.e. Commercials) because film is taking pictures at a certain frame/second rate.

 

[Wannafish]

Your eye/brain can only "update" a visual image at a certain rate. I think it is 6 times per second. Therefore, if something is rotating at a frequency of six times per second (or multiples therof) it will appear to be stopped. As it's rotational frequency increases, it will appear to be going in reverse.

 

[Gatorboy]

</font><font color="blue" class="small">( Your eye/brain can only "update" a visual image at a certain rate. I think it is 6 times per second. )</font>

I think 6 is pretty low.

I found these 2 PhysLink.com links to better explain "Strobing"

Why does your hand look funny when you move it fast in front of the tv or computer screen?

In movies why do wheels of a moving car appear to rotate backwards to the actual direction?

 

[8NTX]

While we're on the topic - has anyone seen the trick that some science museums play with the rotating disc with black marks on it? Put on a record player sort of turntable, starts turning very fast, the black marks turn into colors. Strange.

 

[Gomez]

This also happens in direct sunlight. Your eye/brain act as a low-pass filter. They can only "refresh" so often. The position of the slots at the next refresh will cause the wheel to appear to be reversing.

 

[OkeeDon]

Then there are the new wheels with a bearing-mounted outer wheel that turn independently of the "real" wheel. They keep turning when the car is stopped. They are properly called "bling".

 

[Wannafish]

Gatorboy,

6 may be low...but then I'm probably a little slower than most anyway /forums/images/graemlins/grin.gif.
At least it attempts to give the proper idea of why it happens.

And I like your picture with the cigar better. /forums/images/graemlins/wink.gif

 

[Bird]

</font><font color="blue" class="small">( Then there are the new wheels with a bearing-mounted outer wheel that turn independently of the "real" wheel. They keep turning when the car is stopped. )</font>

Odd you should mention that, Don. I saw my first (and only) one of those on an older model Cadillac about a week or 10 days ago. They kept turning for a loooonnnng time after the car was parked.

 

[BXMMCC]

"While we're on the topic - has anyone seen the trick that some science museums play with the rotating disc with black marks on it?"
And, back in the days of black and white TV, Scientific American magazine had an article on how to use that rotation to get a color picture!