# Homework Help: Definition of no slipping?

1. Dec 25, 2012

### SecretSnow

What does it really mean in physical term? Does it mean no friction? No loss of mechanical energy? Thanks!

2. Dec 25, 2012

### lewando

It means there is plenty of friction. There is no mechanical energy loss. Slipping implies no friction (also no energy loss). Sliding implies friction exists and has been overcome and with a resulting energy loss. Ultimately, how the word is used in the context of the question is to be considered.

3. Dec 25, 2012

### Staff: Mentor

Slipping means that surfaces slide with respect to each other--there is relative motion between the surfaces. There may or may not be friction.

If there is no slipping, then the surfaces do not slide at the point of contact. There may or may not be friction involved. An example would be a ball rolling without slipping. At any instant, the point of contact of the ball is not moving with respect to the surface. There is no energy loss due to friction, since there is no motion of the contact point (at least instantaneously).

4. Dec 25, 2012

### HallsofIvy

In addition, if a wheel or cylinder is "rolling without slipping", during one rotation the center of the wheel will have moved a distance equal to the circumference of the wheel.

5. Dec 25, 2012

### Hurkyl

Staff Emeritus
You do still have rolling friction which, I believe, is static friction occurring at the point of contact?

6. Dec 25, 2012

### Staff: Mentor

Rolling friction is a different kind of thing than ordinary static friction. (At least as I understand it.) But you're right, you could have both static friction and rolling friction even though you are rolling without slipping. (Often rolling friction is ignored.)

7. Dec 25, 2012

### SecretSnow

Thanks all! Then how do I know what does no slipping mean in a question if it may or may not mean there's friction? For now, I guess I can only figure it out through knowing the examples, like the wheel.

8. Dec 25, 2012

### SecretSnow

Why is there no mechanical energy loss if there are plenty of friction?

9. Dec 25, 2012

### lewando

For the "no slipping" case (plenty of friction), imagine standing on a steep incline wearing only rugby boots. There is no slipping, plenty of (static) friction between you and the ground, and no energy loss.

10. Dec 25, 2012

### SecretSnow

Also, if this is the case, why is it that a ball rolling uphill without slipping has friction directed uphill?

11. Dec 25, 2012

### lewando

A ball rolling uphill is going to slow down as a result of gravity. Let's say this ball has a lot of rotational inertia. It is going to want to keep spinning. But it will eventually stop spinning due to a frictional force in the direction opposing its spinning. This force points uphill.

12. Dec 25, 2012

### Staff: Mentor

Friction actually helps the ball go higher up the hill. It helps convert rotational KE into translational KE. Since there's no slipping, no work is done by the friction and thus no mechanical energy is 'lost'.

13. Dec 25, 2012

### SecretSnow

I'm guessing that because v=rw is towards downhill, friction is uphill. Meaning that this friction belongs to the rotating one. However does this apply to only rotating object? Also does it mean that direction of friction of the ball rolling downhill without slipping is pointing downwards too? I noticed that the question in the textbook I use has friction pointed upwards though. I don't know why.

14. Dec 25, 2012

### Staff: Mentor

OK.
What do you mean by that?
Does what apply?
No. When rolling downhill, the ball is speeding up and thus the friction must provide torque to give a rotational acceleration.
The book is correct. Realize that a ball rolling downhill without slipping has a smaller acceleration than if there were no friction. Some of the energy is going into rotational KE.

15. Dec 25, 2012

### SecretSnow

I see. If the friction must provide torque to give the ball a angular acceleration, meaning if it goes downhill the friction is uphill to make it roll, then why does the ball going uphill still have a friction still uphill to make it stop rolling? Although I know that if without friction, the ball will still stop rolling due to gravity What is the rationale of having the friction upwards?

16. Dec 25, 2012

### Staff: Mentor

No slipping means there is no relative velocity between the two surfaces in contact.

17. Dec 25, 2012

### Staff: Mentor

In order to roll uphill without slipping, at any point the velocity must meet the criteria of v = ωr. As the ball slows down, friction must act to decrease the rotational speed accordingly.

18. Dec 25, 2012

### lewando

Consider the direction that the ball rolls when it goes uphill on a frictional surface (say the ball is going from left to right up the hill--therefore it would appear to be rolling in a clockwise manner). At the point of contact between the ball and surface, what direction would you apply a force to make the ball stop rolling?

Actually, without friction the ball will not stop rolling (assuming that it was somehow rolling in the first place--if no friction, the ball will not begin to roll upon contact with the surface).

19. Dec 25, 2012

### SecretSnow

By the way, when I said that friction belonging to the rotational part, I was trying to ask if there's another friction opposing the uphill motion as a whole. Sorry for the poor phrasing!

20. Dec 25, 2012

### SecretSnow

Why won't it roll? If there's a friction uphill to make it stop rolling, then without the frictoon wouldn't it continue rolling?