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How does angular momentum create a perpendicular force?

  1. Sep 27, 2013 #1
    The experiment of a quickly rolling wheel on a string is well known.

    How is this behavior explained in terms of the molecules of an atom? That is to say, why don't the individual atoms of the wheel fall down?
    Last edited by a moderator: Sep 25, 2014
  2. jcsd
  3. Sep 27, 2013 #2
    It is because the atoms are mouseketeers and they took an all for one. and one for all vow to get into to wheel fraternity. Actually the spinning parts of the wheel only have linear momentum, linear momentum at the bottom half is canceled by linear momentum in the top half. The string on the axel of the wheel ties the wheel to a fixed point, if the wheel was to rotate down that would change the linear momentum, but since the linear momentum is conserved, it causes the torques that the professor explained better than I can.
  4. Sep 27, 2013 #3


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    One way to look at the situation is to note that velocity change is the result of an acceleration applied over some period of time. In the case of a wheel in a precessing mode due to wheel rotation and external torque, the velocity change component perpendicular to the plane of rotation of a point on the wheel lags the change in acceleration component perpendicular to the plane of rotation by about 90° as the wheel rotates and precesses.

    The end result is that the reaction of a spinning wheel to an external torque will be a precession about 90° "after" the torque. This effect applies to a single rotor helicopter, so the "cylic", which controls roll and pitch rotation, is advanced by 90° from the pilot input to compensate, and the helicopers roll and pitch responses are precession like reactions to torque inputs from the rotor blades.

    The reason while the wheel doesn't fall down is that the torque due to gravity and the line holding the wheel up is opposed by an equal and opposing torque if the wheel is precessing at a constant and "proper" rate. The other situation is that the free axis end of the wheel will oscillate up and down as the rate of precession increases and decreases.

    The other interesting bit here is that the center of mass of the wheel will tend to stay on the same horizontal plane moving the free axis up or down depending on the movement of the supporting line and how far off from vertical the supporting line is at any moment (the ideal situaion would be a massless supporting line). Look at video #9 on this web page for an example of this:


    Video #18 on that same web page has small masses located at the ends of the spokes that demonstrate the explanation I gave above, but in the video the effect is noted but not explained.

    It doesn't create a force, the precession creates a torque that opposes the torque due to the supporting line and gravitly, so that the wheel doesn't rotate "downwards", and the vertical component of tension in the line is what opposes gravity.
    Last edited: Sep 27, 2013
  5. Sep 27, 2013 #4


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    This might help:

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