Rotating bicycle wheel question

[fluidistic]

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!

 

[tiny-tim]

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 http://64.233.183.104/search?q=cache:p6LO2TP0SXYJ:www.arborsci.com/Data_Sheets/P3-3505_DS.pdf+torque+%2B+%22bicycle+wheel%22&hl=en&ct=clnk&cd=7&client=safari

 

[rcgldr]

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.

 

[fluidistic]

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?).

 

[Phrak]

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.