# Direction of friction

1. Feb 9, 2016

### SDewan

Hi all,
Can anyone please help me out with the direction of friction in a case where there is a smooth spherical body pure rolling on a rough surface? What is the direction of friction?
Also, is it possible to have pure rolling without friction?

Thanks
SD

2. Feb 9, 2016

3. Feb 9, 2016

### BvU

Depends on the inital situation. Most frequent case: if a ball is sliding over a surface, the backwards friction will make it change over to rolling without slipping.

But if it hits the ground spinning like crazy (around a horizontal axis), the forward acting friction will accelerate the motion while adjusting the rate of rotation until there is no more slipping.

And yes: pure rolling without friction is possible. All that is required is $v = \omega R$ (speed = angular speed times radius)

 sorry, missed the 'pure' as in pure rolling (meaning rolling without slipping). Agree with Russ #5.

Last edited: Feb 9, 2016
4. Feb 9, 2016

### sophiecentaur

You can always potential run into trouble if you work with models that are a mixture of ideal and practical. If you have 'no friction' then your model is very limited. You have to work with a very low value of friction - not zero.
You would need to set up that condition with some external force to produce the rolling.

5. Feb 9, 2016

### Staff: Mentor

I think life is simpler: any time an object is rolling without slipping at constant speed, the friction force is zero - regardless of how it got to be that way.

If you want to quibble about exactness of such a scenario, roll a ball down a shallow incline. At some point it will reach a terminal velocity, where friction against the surface is exactly zero.

6. Feb 9, 2016

### sophiecentaur

You're right . . . . . until you have to deal with yet another question on PF about it.

7. Feb 9, 2016