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Why do the spokes on a wagon seam to rotate backwards at certin speeds?

  1. Jan 13, 2009 #1
    This is another thing that I have trouble figuring out. Why do the spokes on a wagon seam to rotate backwards at certin speeds?
    Help!
     
  2. jcsd
  3. Jan 13, 2009 #2

    Vanadium 50

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    Have you ever seen this? I have not. I have seen film of this, but that's a different question.
     
  4. Jan 13, 2009 #3
    You can also see it (same reason - different medium) under fluorescent lights. Long, long ago when Hi-Fi enthusiasts worried about turntable speeds, there were poor-man's strobes that looked something like an optical encoder disk that you could put on the turntable and view under a fluorescent light. Depending on the speed, it went forwards, stood still, or went backwards.

    Now, how you get your Conestoga wagon under a fluorescent light is another question.:smile:
     
  5. Jan 13, 2009 #4

    rcgldr

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    Later and current turntables have a built in strobe and the speed pattern on the side of the turntable.

    One similar effect I've seen in outside in sunlight is an apparent strobe pattern effect from viewing the wheel nuts on a large track at a near parallel to wheel angle (as a passenger with a truck to the passenger side of the viewing car). Although the spacing between nuts is large, I'm guessing the effect is similar to a Zoetrope.
     
  6. Jan 13, 2009 #5

    Vanadium 50

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    The 19th century version of Pimp My Ride?
     
  7. Jan 13, 2009 #6

    jtbell

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    The set they use for "Antiques Roadshow" on PBS looks pretty big. As a bonus, you find out how much your wagon is worth. :biggrin:
     
  8. Jan 13, 2009 #7
    The way it was explained to me goes something like this...

    First of all you have to realize that your eyes are NOT a constant picture feed, but instead they recieve still pictures at regular intervals. These intervals are very short and in effect your brain interprets these series of pics as motion like we see it; sorta like how old film is done or how motion flip books work.

    What happens in order to see this effect your talking about is your eyes take the individual shots at regular intervals but because the wheel or propellor is accelerating at some point the pictures overlap and the reversing effect is observed.
     
  9. Jan 13, 2009 #8

    mgb_phys

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    There's a similar effect giving the stretched wheels on racing cars in early movies.
    The shutter in early film cameras is a slit in a wheel that rotates in front of the film.
    As it turns it exposes the top of the image first and then the next slice below it and finally the bottom.
    So between the time when the top of the wheel is photographed and the bottom the car has moved forward and so the wheel looks like it is stretch backward at the top and forward at the bottom.

    Cartoons draw this effect exaggerated to make the car look fast, but usually draw the wheel forward at the top.

    ps. You can show the 'frame rate' of your eye in a dark room by swinging a flashlight or laser pointer at arms length. You will see a series of lights rather than a continous blur.
    Depending on the lighting etc your eye runs at around 15fps.
     
  10. Jan 13, 2009 #9

    QuantumPion

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    I don't think this is correct. Studies of air force pilots have shown that the human eye can detect changes in light/dark as short as 2 ms (500 fps) and changes in motion as fast as 3-4 ms. The effect of seeing a strobe instead of a blur of a bright object in a dark room is due to how the brain interprets high contrast motion.
     
  11. Jan 13, 2009 #10

    mgb_phys

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    When I said 'eyes' - I should have really said "eye+retina+optic nerve+visual cortex"
    Vision is very complex and what you see depends on a lot of factors - where in your field of view, what contrast, how bright etc.

    The important point is that your eye isn't a continuous system or a digital frame system - it's both/neither.
     
  12. Jan 13, 2009 #11
    Coming from an animation and design perspective, I have always gone off the assumption that the eye cannot detect changes at 24 fps. Any flash animation you see out there is usually running at this speed.

    As to the spokes on a wagon. Quite frankly I have never seen a real-life wagon wheel to comment on that particular instance, but you can notice this effect everyday by watching the hubcaps (er, I'm not sure if thats an Australian term or not, but 'rims' would be an equivalent I think) on most cars. Especially in car ads.

    It is a tricky subject though, because yes the eye only captures so much image at a designated frame rate - but it is also how your own individual brain interprets each of those images. It is like any optical illusion whereas some people will always see the wheel spinning backwards, others will not, and some can choose to see it going backwards and forwards (me being one of them). There is a great illusion out there, of a model standing on one leg - with the other raised in front of it. The model is spinning in circles on the spot. (Supposedly) depending on what side of the brain you use means you will either see the model spinning clockwise, or see it spinning anti-clockwise, or you will be able to see it going both ways and change direction.

    Here she is:

    [​IMG]

    And I would assume that the spokes of your wagon would work on the same principles of this illusion:

    [​IMG]

    Click on the image if it doesnt load - it will take you to a working version. The website might not allow external linking of images.
     
  13. Jan 13, 2009 #12

    mgb_phys

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    I love the dancer illusion, try staring at it and relax-let your eyes go to infinity
    when you focus again you can get it to switch directions.
     
  14. Jan 13, 2009 #13
    Imagine that the wheel has a bright red spot somewhere along the edge. Imagine the spot being right at the top of the spinning wheel.

    Now, your eyes take 'samples' of the spinning wheel, and those samples are shown to your brain at a rapid succession so it looks like it is just a continuous stream of information.

    Now, in 'sample 0' the red spot is at the top of the wheel.
    If the time between two samples is arbitrary, the red spot will appear in a very different location in the next sample. And another different location in the next, etc, so it will appear as a blurry red line instead of a red spot.

    Now imagine that the time between two simples is just right, so that in the first sample 'sample 0' the red spot is at the top. In the next sample, 'sample 1' the spot has rotated nearly one revolution and ended up just before the position of the red spot in sample 0. Then, in the next sample, sample 2, the spot is just before the position of the spot in sample 1, etc. This way, the spot seems to be moving backwards very slowly!

    EDIT
    Cooked up an image to clarify:
    [​IMG]
     
    Last edited: Jan 13, 2009
  15. Jan 13, 2009 #14
    It's easier if you scroll the page down so that you can only see her shadow. Focus on the shadow for a minute. Because it is not fully into view, it can be 'made' to change direction much easier! Once you got the shadow to change direction, scroll back up and the dancer should follow. Pretty amazing illusion I agree!
     
  16. Jan 16, 2009 #15
    The dancer illusion is awesome!
    I am able to switch directions as well as have it appear to go back and forth.

    I heard it said here that it's a left/right brain thing.
    What I wonder is if it's healthy to do this. Maybe it would be a great tool for brain strengthening, or maybe it would fry what few brain cells i have left.

    Anyone know?
     
  17. Jan 16, 2009 #16
    Someone who is autistic can't sense optical illusions generally, why do you think that might be?
     
  18. Jan 16, 2009 #17
    I can only see that dancer spinning clockwise. I've tried everything to make it look like she's spinning counter-clockwise and I just can't seem to do it.
     
  19. Jan 16, 2009 #18

    rcgldr

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    Scroll the picture up until you can just see the dancers foot or just focus on the dancers foot. Without the front + back information it's easier to see imagine the dancer turning the other way. Once you get the foot to appear to be spinning counter clockwise, scroll the picture down slowly or stop looking at just the foot. The dancer will appear to be spinning towards her right if you imagine she is standing on her left leg, and vice versa. Once the model appears to be spinning in a particular direction, my brain "expects" the motion to continue so it's difficult for me to mentally change the direction unless I look away for a brief moment, because to change direction, the dancer has to instantly change which foot she's standing on (and instantly change angular momentum). If I focus on the foot, I can imaging the image changing direction back and forth by imagining either the heel or the toes are always pointed towards me or to the side.

    I see streaks (so do most people), not a series of lights, unless the light source is pulsing.

    Getting back to the OP, is it possible that the view was a side view and the viewer was looking at the spokes of one wheel through the spokes of another, which would create an interference pattern?
     
    Last edited: Jan 16, 2009
  20. Jan 17, 2009 #19
    Because your eyes are broken.

    Nah it's all to do with frame rate. The eye does all sorts of weird things to try to make sense of the world. Has anyone sat on a train and felt like they were moving while another train moved off, only to realise it was them that was moving only. That feels pretty strange.
     
  21. Jan 17, 2009 #20
    Good point.
    Dagda, I recall a specific illusion involving optically switching left/right viewing by a headset mirror arrangement.
    This causes, say, bumps in a road to appear as depressions, and holes to appears as "bumps"; among other odd visual consequences.
    What disturbed me is that I read continued use could result in a lasting(though not permanent) reversal after using the headset.

    These levels of illusions are very powerful.
    Similarly, as mentioned before, I wonder if actively practicing left/right brain switching is a good thing, or not.
     
    Last edited: Jan 17, 2009
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