Rotating bicycle wheel question

  • Thread starter fluidistic
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  • #1
fluidistic
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Hey,
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!
 

Answers and Replies

  • #2
tiny-tim
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Hey,
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!
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.
 
  • #3
rcgldr
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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.
 
  • #4
fluidistic
Gold Member
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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?).
 
  • #5
4,239
1
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?).
Go to the Wikipedia pages on force couple and also torque.
 

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