# Homework Help: Question on Rotational Dynamics

1. Jan 18, 2006

### alamin

1.A uniform Disc of Radius R is rotating in its own plane with angular velocity w when it is placed flat on a rough table. If u the coefficient of sliding friction is independent of velocity show that the time taken for the disc to come to rest is
(3RW)/(4ug). How does the kinetic energy of rotation of the disc vary with time?

2. Jan 18, 2006

### stunner5000pt

ok please show us what work you have done. we arent gonna solve (although we can) your problems for you
also for your problems remember that
$$\Delta K + \Delta U = W_{friction}$$
where K represnets kinetic energy
U represnts potential energy
and W represents work
and delta is the change

3. Jan 18, 2006

### Harmony

May I know what is the formula to calculate work done by frcition? Displacement is given in the case, but the friction force is not.

4. Jan 18, 2006

### stunner5000pt

work done is F d cos(angle)
if angular then d = (angular displacement)

5. Jan 19, 2006

### Psi-String

I thought the work done by the friction force is $$\int \vec{\tau} \cdot d\vec{\theta}$$?

Last edited: Jan 19, 2006
6. Jan 20, 2006

### Tom Mattson

Staff Emeritus
Psi-String,

We appreciate that you are willing to assist, but please do not offer complete solutions to homework questions.

Just to remind everyone, the Homework Help section of the PF Global Guidelines says the following: