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Gyroscopic precession of a bicycle wheel

  1. Jun 28, 2015 #1
    Hello!

    I have recently been looking in to the phenomenon of gyroscopic precession in my free time, and there is a scenario which I am not sure I have fully understood. Here it is:

    Imagine a bicycle wheel with a shaft running through it's center. The shaft is attached to a rope at one of it's sides, and is held upright before it is spun up. If the wheel was not spinning, the force of gravity would simply cause the wheel to start swinging around the attachment point of the rope. However, when the wheel is spinning gyroscopic precession will cause the wheel to stay upright and instead start rotating around the rope, in the horizontal direction. This is supposedly because a force applied to a spinning object acts 90 degrees from the direction it was applied, which in this case causes the wheel to start rotating around the rope.

    My questions are:
    Is the precession occuring because the wheel is starting to change alignment when it starts swinging, and therefore the gravitational force is applied at an angle? In that case, am I right in assuming that the rotation would not start while the wheel is completely verticly upright? Or can a completely upright wheel still precess and rotate?
    rOBQG1f.png mRhxyzl.png
     
  2. jcsd
  3. Jun 28, 2015 #2
    No, the precession occurs because, and at the same instant that, the torque is applied. There does not have to be (and in the idealised case there is not) any rotation in the direction you indicate.
    Yes.
     
  4. Jun 28, 2015 #3

    A.T.

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    Gold Member

    The effect of an external torque on the spin axis is: Rotation of the spin axis around an axis that is perpendicular to both: the current spin axis and the external torque.

    320px-Gyroscope_wheel-text.png

    No, its occurring because an external torque is applied, which changes the angular momentum vector:
    http://hyperphysics.phy-astr.gsu.edu/hbase/rotv2.html
     
    Last edited: Jun 28, 2015
  5. Jun 28, 2015 #4
    Thanks a lot for the answers, they made it a lot clearer.
     
  6. Jun 28, 2015 #5
    There's a simple way to get a handle on why precession occurs.

    Take a wagon wheel because it has rigid spokes, and cut the wheel between each spoke so that it's free from it's neighbors. You can focus attention at the sections that are above, below, and horizontally displaced from the hub at time zero.

    If you refer to your drawing, as a section of rim passes over the top, it's being pushed, or accelerated to the left. This means that as the wheel section reaches a position horizontal with the hub it will tend to be displaced to the left. The section of wheel at the bottom is pushed to the right.

    The forces on the horizontally displaced sections, for the most part, only twist the section about the axis of the spoke.

    All this adds up to overall precession.
     
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