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What's up with spinning tops?

  1. Apr 13, 2008 #1
    Wel, this is just crazy.

    I've noticed something weird about spinning tops. Most of them start loosing speed, then, just before they stop, the direction of rotation is reversed! Why?! What is the force that makes them stop for a instant and then reverse the rotation?

    If you have no idea what I'm talking about, take a look at this video: [Apparently, I can't post URLs to other sites yet, so just go to YouTube and search for a video named "Japanese Koma"]
  2. jcsd
  3. Apr 13, 2008 #2
    The top spins about it's own axis, but there are forces acting on the top (gravity, friction) that apply a torque to the top. This causes the top to precess, so in addition to the spinning about it's rotational axis, the rotational axis itself turns.

    So I think if you could observe the transition in slow motion, you'll find that the top doesn't change directions, but merely that it's precession becomes more pronounced.

    Only a guess though.
  4. Apr 13, 2008 #3
    Uptade: Just noticed something. The direction of rotation is not reversed. The thing is, after the spinning top stops rotating, it "wobbles" a little bit, and that wobbling IS in the opposite direction of the rotation, but if you look at it carefully, the top isn't rotating anymore, it's just wobbling. Now the question is: why does it wobble, and why does it do it in the opposite direction of the previous rotating motion?
  5. Apr 13, 2008 #4
    Ibrits, I hadn't read your message before posting my second one. Seems like our observations agree. Very interesting. Thanks for the answer.
  6. Apr 13, 2008 #5
    Allright, this isn't settled yet.

    How come, WHILE the top is spinning the rotational axis is also spinning in the same direction of the motion? Then when the top stops rotating the rotational axis reverses it's direction of motion. Why does that happen? And what defines the direction of precession?
  7. Apr 16, 2008 #6
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