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Magic Spheres

  1. Dec 11, 2014 #1
    Look at this experiment: Why roll the balls after the collision back to each other? Are they magnetic?
     
  2. jcsd
  3. Dec 11, 2014 #2

    DaveC426913

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    Spoiler! Highliight to read:

    The balls are quite dense. They are running on rails, which means they are rotating much faster than their movement would indicate. There's a lot of angular momentum stored.
    That angular momentum is not lost when they collide and bounce off each other. In effect, as they move apart, they are "skidding" - they've lost traction, but are still rotating. Eventually the rotation gains traction on the rails and the spheres accelerate toward each other again.


    Reviewing the video again, I can see confirmation that I am right. The balls are not perfect; there is an arrow marked on each of them which shows how they are rotating. You can see the arrows in the closeup (as a matter of fact, you can juuuust catch them at the start of the closeup - at 0:19 juuuuust as they disappear into the black reflection at the bottom.) They are rotating the same direction both before and after the collision.
     
    Last edited: Dec 11, 2014
  4. Dec 11, 2014 #3

    Danger

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    Gee, that's complicated. I would have just filed a low spot in the rail. :biggrin:
     
  5. Dec 11, 2014 #4

    DaveC426913

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    :D The first post I made, which I deleted as superfluous, suggested the rails might simply be bowed in the center. But after more examination, I decided the demo was legit, not just a trick.
     
  6. Dec 11, 2014 #5

    Danger

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    You're probably right. My eyes aren't that good any more; I can't really see the spin.
     
  7. Dec 12, 2014 #6
    Hard to say without more data. Is it certain that balls/spheres are made homogeneous ?
     
  8. Dec 12, 2014 #7

    A.T.

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    Look at the sky reflection, right when they collide.
     
  9. Dec 12, 2014 #8

    A.T.

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    The density distribution seems symmetrical, given how uniformly they roll.
     
  10. Dec 12, 2014 #9

    Danger

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    Okay, I've got it now. When I first looked at it, that arrow thingie just didn't quite register in my brain. It took half a dozen more viewings before it did. (I saw it; I just couldn't figure out what it was doing.) Maybe temporal dyslexia affects visual things as well as spoken ones. :redface:
     
  11. Dec 12, 2014 #10

    RonL

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    I'm not sure why I only saw the arrows appear one time ?
     
  12. Dec 12, 2014 #11

    Danger

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    Maybe there's sidespin as well, so they weren't in a visible spot earlier. Originally, once I realized that they were arrows, I suspected that they might have been digitally added to indicate spin, but that doesn't seem to be the case.
     
  13. Dec 12, 2014 #12

    A.T.

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    Because they cross the upper bright part only one time. But you see them also at 0:17 on the left ball, in the lower right part.
     
  14. Dec 12, 2014 #13

    RonL

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    Thanks, yes:)
     
  15. Dec 12, 2014 #14

    DaveC426913

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    Did no one read my spoiler? It describes the arrows.
     
  16. Dec 12, 2014 #15
    I just read the OP's "are they magnetic?", watched once, then read your spoiler, then watched again and it all made sense.

    Once I realised the balls were rolling on rails the dramatic pushes and attractive reactions all fell into perspective.
     
  17. Dec 12, 2014 #16

    Danger

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    I read it, and once I actually realized that they were physically present as opposed to CGI it made sense.
     
  18. Dec 12, 2014 #17

    RonL

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    I can't understand what highlight to read, is supposed to do ?
    Your next post said you deleted the first one.
    Now you ask if no one read it ? I'm confused other than to say post #4 you say it's not a trick o_O:)
     
  19. Dec 12, 2014 #18
    I didn't get it at first either, just click and drag the mouse like you are selecting text to copy or quote.
     
  20. Dec 12, 2014 #19

    RonL

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    Thanks, I finally figured it out :) I'm pretty simple minded when it comes to finding text that's not visible. :D
    I did slide the mouse on the text that was visible and all that came up was +quote or reply :oops:
     
  21. Dec 13, 2014 #20
    I believe this man is right. ( and I didn't say that because he looks like moses.)
     
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