# Help in understanding a question about a pebble and a wheel and friction...

Tags:
1. May 20, 2017

### Buffu

1. The problem statement, all variables and given/known data

A wheel of radius R rolls along the ground with velocity V. A pebble is carefully released on top of the wheel so that it is instaneously at the rest on the wheel. Show that in the case $V < \sqrt{Rg}$ and the coefficient of friction is $\mu = 1$ the pebble starts to slide when it is rotated through an angle given by $\theta = \arccos (1/\sqrt{2}(V^2/Rg)) - \pi/4$

2. Relevant equations

3. The attempt at a solution

I would attempt at the solution if only I understand what its by "sliding" in the question ? Does it mean that the pebble will fly off the wheel ? or its accelaration is zero ?

Last edited by a moderator: May 20, 2017
2. May 20, 2017

### Staff: Mentor

Can you post a picture of the problem? It's kind of hard to help when I can't visualize the situation. Thanks.

3. May 20, 2017

### Buffu

There was no picture in the book.

4. May 20, 2017

### ehild

Initially, the pebble stays in rest on the wheel moving together with the rim. When the wheel turns by angle theta, then the pebble starts to slide on the surface downward. Draw the free-body diagram.

5. May 20, 2017

### Buffu

When it slide is it still on contact with the surface ?

6. May 20, 2017

Yes.