Rotating bicycle wheel question

In summary: They are related concepts and will help you understand how forces act on an object to produce rotational motion. In summary, the conversation revolved around the concept of torque and how it affects the rotation of an object. The person was initially confused about the direction in which to apply force to change the plane of a rotating bicycle wheel, but eventually understood that torque and the direction of the applied force were key factors. They also discussed the idea of a force couple and its relationship to torque.
  • #1
fluidistic
Gold Member
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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!
 
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  • #2
fluidistic said:
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 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
 
  • #3
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
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
fluidistic said:
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.
 

1. What is the Rotating Bicycle Wheel Question?

The Rotating Bicycle Wheel Question is a physics thought experiment that explores the concept of inertia and angular momentum. It involves a person sitting on a rotating bicycle wheel and trying to turn the wheel while holding onto the handlebars.

2. Why is this question important?

This question is important because it helps us understand the principles of rotational motion and the effects of angular momentum. It also highlights the importance of conservation of angular momentum in various real-life scenarios, such as figure skating and spinning tops.

3. What happens when the person on the bicycle wheel turns the handlebars?

When the person turns the handlebars, they are trying to change the direction of the angular momentum. This results in a torque being applied to the wheel, causing it to precess or wobble. This is because the angular momentum of the wheel wants to remain in its original direction due to inertia.

4. Can the person on the bicycle wheel stop the precession?

Yes, the person can stop the precession by applying a counter-torque in the opposite direction of the precession. This will cancel out the torque and allow the wheel to continue rotating in its original direction.

5. How does this question relate to other physics concepts?

The Rotating Bicycle Wheel Question connects to other physics concepts such as conservation of angular momentum, torque, and inertia. It also has applications in gyroscopic motion, which is used in various devices such as gyroscopes and navigational instruments.

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