# Friction and rolling

1. Dec 22, 2011

### hokhani

when a roller is rolling on a flat surface with a constant speed, how is about the friction force?
is there any friction forces or not?

2. Dec 22, 2011

### Staff: Mentor

What are your thoughts? The term "constant speed" is a key in this question...

3. Dec 22, 2011

### Curl

constant speed doesn't say much - it could be a forced motion.

if the roller is rolling without slipping, there is static friction with does no work.

4. Dec 22, 2011

### Staff: Mentor

Ah, good point. Didn't think of that possibility.

5. Dec 23, 2011

### hokhani

thanks all
let me clarify my question:
consider a roller without slipping(a disk) which it's center has a constant speed.
if there were any friction force on a roller during rolling then the disk can't have a constant speed(due to friction the velocity is decreasing). on the other hand the torque(due to friction) should increase it's angular velocity.
so i think there is a discrepancy here.

6. Dec 23, 2011

### Staff: Mentor

If something is just rolling without slipping along a horizontal surface, the static friction will be zero. Static friction would be non-zero if it were accelerating.

(This ignores losses due to deformation and rolling friction, of course.)

7. Dec 23, 2011

### hokhani

ok
thanks

8. Dec 23, 2011

### Staff: Mentor

Have you rolled a marble along a flat surface? What about trying different flat surfaces?

9. Dec 23, 2011

### rcgldr

I'm guessing your thinking of rolling resistance, as opposed to static friction. Wiki article about rolling resistance:

http://en.wikipedia.org/wiki/Rolling_resistance

10. Dec 23, 2011

### Lsos

There is pretty much always friction.

11. Dec 23, 2011

### Curl

When the disk is freely rolling without slipping at constant velocity, the friction is zero. If your disk rolls onto a frictionless surface, there will be no difference in how it moves - it will still appear to be rolling without slipping because rw=v i.e. they are matched perfectly.

Friction only comes into play if there is any acceleration.