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Two types of precession?

  1. Apr 26, 2009 #1
    It occured to me recently that there are two forms of precession.

    If you hold a spinning bicycle wheel and force the axis of rotation to tilt on its side, this will induce a torque tending to spin the wheel about a vertical axis. This is in accord with the fact that the change in angular momentum is in a vertical direction.

    On the other hand, in the experiment where a spinning bicycle wheel is supported on one end of the axis by a rope and undergoes precession, the direction of the induced rotation is not the same as the direction of the induced torque (which changes over time).

    I have always understood the second experiment as an example of precession. Would one put the first example into this same category?
  2. jcsd
  3. Apr 26, 2009 #2


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    I can think of two other forms of precession although you can't exemplify them with the spinning bicycle wheel.

    There is orbital precession where the semimajor axis of an orbiting planet changes over time. The deviation of Mercury's orbital precession from Newtonian predictions helped confirm Einstein's GR.

    The other occurs when an object is freely spinning with no torques but the axis of rotation is not through a principle axis. Imagine a rectangular box in freefall (initially) rotating about the diagonal through opposite corners. What occurs is that the axis of rotation is not parallel to the direction of the angular momentum. The angular momentum is conserved but the axis of rotation will precess about the direction of the angular momentum. What is more the axis of rotation relative to the object will oscillate between principle axes.
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