# Homework Help: Friction during pure rolling up an inclined plane.

1. Jan 31, 2012

### AlchemistK

1. The problem statement, all variables and given/known data
A cylinder is pure rolling up an inclined place. It stops momentarily and the rolls back. In what directions is the force of friction directed during the journey?

3. The attempt at a solution

While going up, the cylinder rotates in the anticlockwise direction, and so the friction acts up the plane. On the other hand, while rolling down, the cylinder rotates clockwise, and so the friction acts downward.
Which of those statements is wrong and why?

(As I started thinking, another query arose in my mind; since it is pure rolling, shouldn't the only friction be static friction? And so should that even have a direction?)

2. Jan 31, 2012

### SammyS

Staff Emeritus
Yes it is static friction. It also has a definite direction.

If the incline were frictionless, would the cylinder's rotational rate change upon going up and down the incline?

3. Jan 31, 2012

### AlchemistK

Does static friction has a direction opposite to relative velocity, just like kinetic friction?

Would the cylinder rotate in the first place? There would be no torque to get it started.

And the book says that the answer is that the direction of friction is always up the plane, why is that?

4. Dec 12, 2012

### alex.hs

The actual reason for friction to act in pure rolling is :
g sine ( theta ) acting along the incline causes an acceleration on the point of contact of the body. But the body is supposed to be in pure rolling I.e at the point of contact it should have zero velocity and zero acceleration. Since the nature of frictional force is to oppose relative motion making relative velocity zero..it opposes the mg sine ( theta) force and when this friction becomes equal to that value the body will be in pure rolling.

Coming back to the original question...
Whether the body is moving upwards or downwards...mg sine ( theta ) always acts down the incline so frictional force, to oppose the relative velocity , will always act in the direction opposite to it i.e up the incline..
And if the incline is frictionless there is no case of pure rolling unless an external force is applied opposing mg sine ( theta) force
Hope u got d concept...

Last edited: Dec 12, 2012
5. Dec 12, 2012

### SammyS

Staff Emeritus
Hello alex.hs . Welcome to PF !

Do you realize that you've responded to a thread that's over 10 months old ?

6. Dec 14, 2012

### alex.hs

I guess I did not observe..:shy:
We are actually learning that topic right now...so when I saw it, I just wanted to answer it.
Anyways....thank you SammyS for letting me know..