I Rolling with slipping sliding friction...

1. Oct 4, 2016

FallenApple

I'm a bit confused. We all know that rolling with slipping is associated with kinetic friction.

But Is that friction due to the traslational motion of the center of mass or just the spinning motion?

If friction exists because of the translational motion, then in theory, I can slowly lower a spinning disk on its edge onto a frictional surface, and it should just stay there, spinning, since there is no sliding motion.

2. Oct 4, 2016

Staff: Mentor

Kinetic friction acts between surfaces that slip with respect to each other. It's the relative motion of the surfaces that matter, not the motion of the center of mass.

3. Oct 4, 2016

FallenApple

Ok got it. Thanks.

Also I was working on a problem, a ball has initial velocity to the left and is rotating in the clockwise direction. So there is going to be a friction force to the right, slowing down the roll and also the cm velocity. Eventually, the velocity of the cm is 0 at and instant, but the ball still has some spin clockwise.

Would you say in this case that the ball's cm velocity will immediately increase from 0 and point to the right?

If so, this problem is a bit odd. Sure, if the ball moves back to the right, angular momentum is still conserved.

But the linear momentum isn't.

It seems like the friction is a restoring force.

4. Oct 4, 2016

PeroK

In this case friction is an external force and neither linear nor angular momentum of the ball is conserved.

5. Oct 4, 2016

FallenApple

http://web.mit.edu/8.01t/www/materials/ExamPrep/exam03_sol_f13.pdf

In the link, for question 1, they solved a problem where they needed to find the speed of the center of mass after it rolls without slipping. They used conservation of angular momentum even though there was friction.

6. Oct 4, 2016

PeroK

That's without slipping. This is a special case where friction does no work. In fact, if a ball is rolling without slipping and moves onto a frictionless surface it will keep rolling without any change to its motion.

Friction is needed to establish the rolling without slipping equilibrium but after that has been established it does no further work on the object.

PS I looked at the problem in the pdf. They don't and can't use conservation of angular momentum to solve problem 1. Friction provides an external torque until rolling without slipping is established.
.

Last edited: Oct 4, 2016
7. Oct 4, 2016

FallenApple

But in that problem in the link, the cylinder started slipping before it finally got pure roll. The Li is based on the before picture and the Lf is based on the after picture, where slipping happened sometime in between. So in that problem, there was work done in getting the cylinder to pure roll state.

In the problem I mentioned with the ball moving backwards, the ball still needs to go though a time frame where the ball is slipping.
But if I pick the origin on the surface of the horizontal ground, the f is parallel to the lever arm, so there is no external torque.

So are you saying that the friction does positive work to establish the pure roll, and that gets unleashed in terms of constant pureroll motion? Kinda like a spring?

8. Oct 4, 2016

PeroK

I added a PS above. I'm not sure now what problem you mean. When a ball stops linearly but still has rotation, it's the rotation that gets it moving backwards and slipping in the opposite direction starts until rolling without slipping is achieved. Slipping in the sense of over-rotating.

9. Oct 4, 2016

Staff: Mentor

No problem.
It's perfectly OK to use conservation of angular momentum as long as you pick the correct point. For example, the point of contact with the surface. Friction exerts no torque about that point.

10. Oct 4, 2016

Staff: Mentor

As is often the case, there are multiple ways to solve this problem. You can treat this using dynamics, where the friction force creates a translational deacceleration and a rotational acceleration until the conditions for rolling without slipping are met. (Friction certainly does work during slipping, as already noted by you and PeroK.)

Or you can use conservation of angular momentum.

11. Oct 5, 2016

A.T.

And neither do gravity or the normal force, in the idealized case.

Last edited by a moderator: Oct 5, 2016
12. Oct 5, 2016

Staff: Mentor

Right. Those forces cancel out.