How Everything Works
How Everything Works How Everything Works

Question 539

I've seen tops that rest with their large parts down but that flip up onto their handles when you spin them. What is the reason that they have a different equilibrium when they are spinning versus when they are not? — CH, Renton, WA
While I'm not an expert on these "tipple tops," I believe that I understand how they work. These tops have large round heads and look like wooden mushrooms. When you hold the handle (the mushroom's stem) and spin it with its head down, it quickly flips over so that it spins on its handle. The flipping is caused by a torque that friction exerts on the top's round head as the tops surface slides across the table. If the top were perfectly vertical as it spun on its head, friction between the top and the table would exert a torque (a twist) on the top that would simply slow the top's rotation. But when the top isn't perfectly vertical, the torque that friction exerts on it does more than slow its rotation. This torque also causes the top to precess (change its axis of rotation) in such a way that the top's handle gradually becomes lower and the top's head gradually becomes higher. Eventually, the top's axis of rotation inverts completely so that it begins to rotate on its handle. Once that happens, the precession stops because the handle is too narrow for anything but the slowing effects. Only when the top stops spinning does it shift from this dynamically stable arrangement (handle down) to its statically stable arrangement (head down).

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