Rear Wheel Friction: Cycling on an Incline

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Discussion Overview

The discussion revolves around the direction of friction on the rear wheel of a bicycle moving down an incline, considering two scenarios: when the bicycle is being pedaled and when it is not. Participants also explore the implications of these scenarios on flat surfaces.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • Some participants inquire about the direction of friction on the rear wheel when the bicycle is moving down an incline both while pedaling and not pedaling.
  • One participant clarifies the terminology, noting that "paddling" refers to a boat, while "pedaling" is the correct term for a bicycle.
  • A participant suggests visualizing the forces on the wheel using a sliding paper analogy, explaining that when pedaling, the frictional force on the back tire is directed forwards due to the backward slip tendency of the tire.
  • Another participant points out that when not pedaling, there is a small backward force acting on the bottom of the tire due to frictional torque opposing the wheel's rotation.
  • There is a discussion about the Newton's Third Law pair of forces, indicating that the direction of friction can depend on whether one is considering the force exerted by the tire on the surface or vice versa.
  • One participant references a library resource on the direction of friction in relation to driving and non-driving wheels of a car, suggesting a parallel to the bicycle discussion.

Areas of Agreement / Disagreement

Participants express varying views on the direction of friction under different conditions, and the discussion remains unresolved with multiple competing perspectives on the topic.

Contextual Notes

Some assumptions about the conditions of motion, such as the incline's angle and the speed of the bicycle, are not explicitly stated, which may affect the analysis of friction direction.

paras02
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1. Pls tell me the direction of friction on the rear wheel of a cycle moving down on an incline plane in both the cases that are when cycle is being paddled and without any paddling ?
 
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Is the cycle moving without any pedalling
 
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paras02 said:
1. Pls tell me the direction of friction on the rear wheel of a cycle moving down on an incline plane in both the cases that are when cycle is being paddled and without any paddling ?

What is your answer to this question if there is no incline?
 
Chestermiller said:
What is your answer to this question if there is no incline?
when pedalling in forward and when not pedalling in backward
 
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Paddling is moving a boat using a paddle.

A bicycle is moved by turning it's pedals so the word is pedalling.
 
Oops sorry
 
Since we're dealing with words... The word is its, in the context of its pedals! There should be no apostrophe, either in English English or in American English.

I think you can get a good intuitive feel for the forces on the wheel by imagining that you're riding the bike on layers of paper which can slide over each other if the tangential stress is above a certain value. If you're pedalling the bike hard and the paper starts to slip, it will slip backwards - visualise it or try it - because of the frictional force from the back tyre (tire). So the paper (or, without paper, the road surface) will exert a forward force on the tyre (Newton's Third Law). This is the force that accelerates the bike, or, if moving at a steady speed, balances the backwards force of air resistance on the bike.

But (you might argue) doesn't friction always oppose motion? Not exactly. It opposes relative motion between surfaces. The bottom of the back tyre is trying to slip backwards on the road, when you're pedalling. The frictional force on the bottom of the back tyre is therefore forwards.

When you're not pedalling there is a small backwards force from the road acting on the bottom of the tyre. To understand why, remember that there is a small frictional torque opposing the turning of the wheel. In order to keep the wheel turning as the bike descends the hill, there must be a small backwards force on the bottom of the back tyre.

Hope this helps.
 
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There is a Newton third law pair of forces involved with friction. The direction of friction depends if you mean the force the tire's contact patch exerts on the surface of the incline, or the force that the surface of the incline exerts on the tire's contact patch.
 
Thank you guys
 
  • #10
hi paras02! :smile:

"In particular, the friction from the road on the driving or braking wheels of a car is in the same direction as the acceleration or braking, but the friction on the non-driving or non-braking wheels of a car is in the opposite direction."

see the pf library on direction of friction for more detail :wink:
 
  • #11
tiny-tim said:
"In particular, the friction from the road on the driving or braking wheels of a car is in the same direction as the acceleration or braking, but the friction on the non-driving or non-braking wheels of a car is in the opposite direction."
"Friction" or "rolling resistance" for the non-driving tires? There is some friction force at the axles of the non-driving wheels (but that is effectively a small amont of braking), which will also mean some friction force at the tires.
 
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