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Rotating Object Changing Direction

  1. Jun 12, 2010 #1
    I recently saw a demonstration where an elongated object (not a top) was rotated. The demonstrator applied some force to spin it but also pushed down on the side of the object, causing it to wobble as it spun. It then slowed down, stopped, and changed direction, spinning in the other direction. If it is relevant, I think there were masses placed at either end of the object. Does anyone know what causes this?

  2. jcsd
  3. Jun 15, 2010 #2

    Andrew Mason

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    This can occur if the wobble or precession is opposite to the direction of spin. When it slows down, the wobble increases and it eventually falls over on its sidee. When this occurs, the friction between the spinning object and the surface it is on causes the object to roll around on the surface. Since the direction of the spin is opposite to the direction of the precession, the object changes direction.

  4. Jun 15, 2010 #3


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    It's not precessing. It's not spinning fast enough for any significant gyroscopic effects.

    I've seen one of these made into a cast plastic toy already. But yes, it has to do with friction and the object's odd geometry. I wish I could remember what they called them, so I can look up a video, or at least some images for others to see.

    Edit: Ha! Found. it. It's called a Rattleback.
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