# Rotating bicycle wheel question

Gold Member
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!

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tiny-tim
Homework Helper
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!

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

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.

See this.

rcgldr
Homework Helper
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.

Gold Member