How can I use the right hand rule to find torque in bicycle wheel rotation?

AI Thread Summary
The discussion focuses on using the right hand rule to determine torque in bicycle wheel rotation, emphasizing the importance of identifying the point of force application. Two scenarios are considered: one where friction prevents slipping and another where it does not. The direction of friction is clarified as opposing the relative motion of surfaces in contact, which is crucial for understanding torque. The participants agree that if the force is applied to the right, friction must act to the left to prevent slipping. Overall, friction plays a vital role in the wheel's rotation and torque calculation.
ReCryptic
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Homework Statement
Suppose you stand up a bicycle wheel and push on its lower spoke in the direction shown which way will it roll? Explain your reasoning and assume there is friction
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image.jpg

I tried to use the right hand rule to find torque but I’m not sure to go from there.
 
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Do you think it matters if the applied force is enough to overcome the friction and make the tire slide?
 
Hi ReCryptic. Do free body vector diagram showing all the forces at the moment you apply force.

There are at least a couple of possible scenarios:
1. Where the friction force on the tire does not allow the wheel to slip.
2. Where the friction force on the tire is insufficient to overcome the applied force of your hand and the tire moves relative to the surface.

The torques on the wheel will depend on where on the spoke you apply the force. So you will need to identify where the applied force is located (i.e. distance to hub and distance to surface) to work out the torques. hint: the right hand rule to work out the torque directions is really not that important for this question.

AM
 
phinds said:
Do you think it matters if the applied force is enough to overcome the friction and make the tire slide?
I think we are supposed to assume that the wheel doesn't slip.
 
Andrew Mason said:
Hi ReCryptic. Do free body vector diagram showing all the forces at the moment you apply force.

There are at least a couple of possible scenarios:
1. Where the friction force on the tire does not allow the wheel to slip.
2. Where the friction force on the tire is insufficient to overcome the applied force of your hand and the tire moves relative to the surface.

The torques on the wheel will depend on where on the spoke you apply the force. So you will need to identify where the applied force is located (i.e. distance to hub and distance to surface) to work out the torques. hint: the right hand rule to work out the torque directions is really not that important for this question.

AM
How do I know which direction friction is if I don’t know the direction of motion?
 

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ReCryptic said:
How do I know which direction friction is if I don’t know the direction of motion?
Well, you just said to assume that it doesn't slip so how many directions are left?
 
phinds said:
Well, you just said to assume that it doesn't slip so how many directions are left?

So then friction has to be towards the left.

The wheel rotates clockwise because the force is applied to the left and friction acts opposite to that applied force. Am I right?
 
ReCryptic said:
How do I know which direction friction is if I don’t know the direction of motion?
Friction does not simply "oppose motion". It opposes relative motion of the surfaces in contact.
See https://www.physicsforums.com/insights/frequently-made-errors-mechanics-friction/
Without friction, what would be the relative motion of those surfaces? Which way must friction act on the tyre to prevent that?
 
ReCryptic said:
So then friction has to be towards the left.

The wheel rotates clockwise because the force is applied to the left and friction acts opposite to that applied force. Am I right?
?
You just said that the friction force acts towards the left AND the force applied acts to the left (despite showing a hand pushing to the right).
 
  • #10
phinds said:
?
You just said that the friction force acts towards the left AND the force applied acts to the left (despite showing a hand pushing to the right).
I just realized I typed that wrong. The force is applied to the right and friction acts to the left.
 
  • #11
phinds said:
?
You just said that the friction force acts towards the left AND the force applied acts to the left (despite showing a hand pushing to the right).
@ReCryptic may have meant "applied to the left" in the sense of being applied on the left, i.e. the point of application is to the left of the wheel.
 
  • #12
ReCryptic said:
I just realized I typed that wrong. The force is applied to the right and friction acts to the left.
In that case, yes you have it right in post #7
 
  • #13
haruspex said:
Friction does not simply "oppose motion". It opposes relative motion of the surfaces in contact.
See https://www.physicsforums.com/insights/frequently-made-errors-mechanics-friction/
Without friction, what would be the relative motion of those surfaces? Which way must friction act on the tyre to prevent that?
Without friction, the wheel would just slip on the surface, but not really move.
 
  • #14
ReCryptic said:
Without friction, the wheel would just slip on the surface, but not really move.
It would not rotate, but the surfaces in contact would slide past each other. Which way would the wheel slide? Which direction must the frictional force be to prevent that?
 
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  • #15
Wouldn't the wheel slide towards the right? Then friction would have to act towards the left.
 
  • #16
ReCryptic said:
Wouldn't the wheel slide towards the right? Then friction would have to act towards the left.
Yes.
 
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  • #17
haruspex said:
Yes.
Okay thank you.
 

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