# Direction of friction

1. Nov 3, 2012

### paras02

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 ?

Last edited: Nov 3, 2012
2. Nov 3, 2012

### Nstraw

Is the cycle moving without any pedalling

Last edited: Nov 4, 2012
3. Nov 3, 2012

### Staff: Mentor

What is your answer to this question if there is no incline?

4. Nov 3, 2012

### Nstraw

when pedalling in forward and when not pedalling in backward

Last edited: Nov 4, 2012
5. Nov 4, 2012

### AJ Bentley

A bicycle is moved by turning it's pedals so the word is pedalling.

6. Nov 4, 2012

Oops sorry

7. Nov 4, 2012

### Philip Wood

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.

Last edited: Nov 4, 2012
8. Nov 4, 2012

### rcgldr

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.

9. Nov 9, 2012

### paras02

Thank you guys

10. Nov 10, 2012

### tiny-tim

hi paras02!

"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

11. Nov 10, 2012

### rcgldr

"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.

Last edited: Nov 10, 2012