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Rotating bicycle wheel question

  1. Oct 31, 2008 #1


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    Today I assisted to a lecture I didn't understand in Physics I.
    I've asked to a helper-teacher a question and he made me much more confused.
    Say I have a rotating bicycle wheel in my hands (my hands don't influence the wheel's rotation) in front of me such that the wheel is in a vertical plane only. If I want to change the plane from the vertical to the horizontal, in what direction must I apply a force on the wheel? The helper told me "down". I said OK and left the class. While I was leaving I was completely mixed up. Does that mean that if the wheel rotates at say 10000 Hz, I wouldn't be able to inclinate it from vertical to horizontal, even if the wheel is in the air and that I put all my weight on it? It would inclinate a very bit and at last it would fall off the floor. But this implies that the wheel can get stuck in the air for a while if it rotates at a high angular speed and in a vertical plane. Is that true? I just can't believe it!
  2. jcsd
  3. Nov 1, 2008 #2


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    Hey fluidistic! :smile:

    (assister? mais non, "assist" means "help" … you attended a lecture :wink:)

    You can turn the wheel … but it may need a lot of force (torque) … and you may have to rotate yourself ("do a cartwheel") to compensate. :smile:

    See this.
  4. Nov 1, 2008 #3


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    You need to applied torque perpendicular to the axis, not a force, with the response being 90 degrees out of phase with the torque you apply. In this case you need to apply a yawing torque (turning around while holding the wheel will do the trick), and it will respond with a roll reaction.
  5. Nov 1, 2008 #4


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    Thanks both for your replies.
    When I turned off the computer yesterday I realized I was wrong in something : I thought that the wheel would preserve its vertical plane but also its position in space which is untrue. Of course if I let the wheel fall off it will fall and not stay in the air as I thought. But I understand now that it will fall conserving its vertical plane and when it will reach the floor it will begin to roll.
    I'm getting the concept, but I must read more on this. Especially on couple (torque?).
  6. Nov 2, 2008 #5
    Go to the Wikipedia pages on force couple and also torque.
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