# Homework Help: Cylinder's Rotational Motion

1. Nov 18, 2006

### Soaring Crane

A cylinder is observed to be rolling freely at a constant speed on a horizontal surface. Which of the following statements is true?

a.It is possible that the cylinder is both rolling and sliding at the same time, i.e. that v, the speed of the centerpoint, is not = to rw.

b.If the cylinder is rolling without slipping, there must be a non-zero friction force exerted by the surface.

c.It is not possible for the cylinder to roll unless friction is present.

d.Even if the surface has friction, the cylinder cannot roll without slipping.

e.If friction is present it is not possible for the motion to occur without loss of energy.

Would the answer be b.? For rolling without slipping, static friction must be present.

Thanks.

2. Nov 18, 2006

### Staff: Mentor

Careful. Note that the surface is horizontal.

Last edited: Nov 18, 2006
3. Nov 18, 2006

### turdferguson

If a block is accelerated on a surface with friction and then hits a frictionless surface, it will continue with its velocity forever. A wheel on a horizontal surface only needs friction to accelerate

4. Nov 18, 2006

### Soaring Crane

On a horizontal surface, can't rolling without slipping occur with and without friction?

Also, is it possible, as in choice A, for slipping and rolling to occur simultaneously like driving across an icy road?

5. Nov 19, 2006

### Staff: Mentor

If you mean "Can rolling without slipping occur even when the surfaces are frictionless?", then yes, if the angular and translational speeds comply with $v = \omega r$.

But even if the surfaces are not frictionless, what friction force is needed to maintain rolling without slipping on a horizontal surface?

Yes, rolling and slipping can certainly occur together. Nowhere does it say that the cylinder necessarily rolls without slipping.

Since the cylinder rolls at a constant speed, what can you say about the friction force that acts on it?

6. Nov 19, 2006

### Soaring Crane

Isn't there a rolling friction? Shoudn't it be kinetic friction if the horizontal surface is not frictionless?

7. Nov 19, 2006

### Staff: Mentor

Let me rephrase my question: Since the cylinder, we are told, is moving at a constant speed, what must be the net force on it? Given this, what must be the friction force on it?

8. Nov 19, 2006

### Soaring Crane

If there is constant speed, the acceleration is 0, so the net force must be 0 N?

9. Nov 19, 2006

### Staff: Mentor

Right. And since the cylinder is rolling freely (meaning: nothing is pushing or pulling it), what must be the friction force?

10. Nov 19, 2006

### Soaring Crane

The friction force would also be zero since it is the only other possible component in the horizontal direction? Does this mean that choices b-d are false?

11. Nov 20, 2006

Right.
Yes.