1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

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

    User Avatar
    Gold Member

    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

    User Avatar
    Gold Member

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

    DaveC426913

    User Avatar
    Gold Member

    :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

    User Avatar
    Gold Member

    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.

    User Avatar
    Science Advisor
    Gold Member

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

    A.T.

    User Avatar
    Science Advisor
    Gold Member

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

    Danger

    User Avatar
    Gold Member

    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

    User Avatar
    Gold Member

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

    Danger

    User Avatar
    Gold Member

    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.

    User Avatar
    Science Advisor
    Gold Member

    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

    User Avatar
    Gold Member

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

    DaveC426913

    User Avatar
    Gold Member

    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

    User Avatar
    Gold Member

    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

    User Avatar
    Gold Member

    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

    User Avatar
    Gold Member

    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.)
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Magic Spheres
  1. A magic can (Replies: 12)

  2. Velocity sphere (Replies: 3)

  3. Due spheres (Replies: 0)

  4. Sphere on a Flat Plane (Replies: 3)

Loading...