Rolling with slipping sliding friction....

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Discussion Overview

The discussion revolves around the concepts of rolling motion, slipping, and the role of friction in these dynamics. Participants explore the relationship between translational and rotational motion, the effects of kinetic friction, and the conservation of momentum in various scenarios involving rolling objects.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • Some participants question whether kinetic friction arises from translational motion or spinning motion, suggesting that a spinning disk could remain stationary if placed on a frictional surface without sliding.
  • It is noted that kinetic friction acts between surfaces that slip relative to each other, emphasizing the importance of relative motion over the center of mass motion.
  • A scenario is presented where a ball with an initial leftward velocity and clockwise rotation experiences a friction force that slows both its roll and center of mass velocity, raising questions about the subsequent motion of the ball.
  • Participants discuss the implications of friction as a restoring force and its role as an external force affecting linear and angular momentum conservation.
  • There is mention of a problem from a linked resource where conservation of angular momentum is applied despite the presence of friction, leading to a discussion on the conditions under which this is valid.
  • Some participants argue that friction is necessary to establish rolling without slipping but does no further work once that state is achieved, while others contend that work is done during the transition from slipping to pure rolling.
  • Different approaches to solving problems involving rolling and slipping are suggested, including dynamics and conservation of angular momentum, with emphasis on selecting appropriate reference points for analysis.

Areas of Agreement / Disagreement

Participants express multiple competing views regarding the role of friction, the conservation of momentum, and the conditions under which different principles apply. The discussion remains unresolved with no consensus reached on these points.

Contextual Notes

Participants highlight the complexity of the interactions between translational and rotational dynamics, the conditions necessary for rolling without slipping, and the implications of external forces like friction on momentum conservation. There are unresolved assumptions regarding the nature of slipping and the effects of friction in various scenarios.

FallenApple
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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.
 
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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.
 
Doc Al said:
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.
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.
 
FallenApple said:
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.
In this case friction is an external force and neither linear nor angular momentum of the ball is conserved.
 
PeroK said:
In this case friction is an external force and neither linear nor angular momentum of the ball is conserved.
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.
 
FallenApple said:
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.

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.
.
 
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PeroK said:
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 it's 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.

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?
 
FallenApple said:
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.
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.
 
FallenApple said:
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.
No problem.
PeroK said:
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.
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
FallenApple said:
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.
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
Doc Al said:
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.
And neither do gravity or the normal force, in the idealized case.
 
Last edited by a moderator:
  • #12
A.T. said:
And neither do gravity or the normal force, in the idealized case.
Right. Those forces cancel out.
 

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