# Homework Help: Constant angular velocity

1. May 23, 2007

### PJ81

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

A coin is placed on a record turntable at a distance of 10cm from the turntable axis. The coefficient of static friction between the coin and the turntable is 0.21. If the turntable starts from rest with a constant angular acceleration of 1.2 rad/s2, how much time passes before the coin begins to slip?

2. Relevant equations

Ff= coefficient of friction x Radius

3. The attempt at a solution

Ff= 0.21 x 0.1 = 0.021Nm
Am I on the right track? where to from here,I can't find any examples in my textbook to help me any input would be appreciated

2. May 23, 2007

### Dick

The coin is in circular motion. What's the relation between it's acceleration and the radius and the angular velocity? I don't think you are looking very hard in your book. In fact as far as "Ff=coefficient of friction x Radius" I don't think you looked in your book at all. You just made that up.

3. May 23, 2007

### PJ81

Sorry, I didn't make it up , I stuffed up as the letter R is normal force in this case not the radius. Anyway, is it as simple as the coin starts moving at the point where the tangental force is greater than the frictional force?

Fat x t = Ff
t=Ff/Fat

Fat=alpha x Radius x m Ff= coefficient friction x m x g
=1.2 x0.1 x m = 0.21 x 9.81 x m
=0.12 x m [N] = 2.06x m [N]

t= 2.06 x m / 0.12 x m
=17.16seconds

4. May 23, 2007

### Dick

It will start moving when it's total acceleration times its mass exceeds the force of static friction. But there are two components to the acceleration. There is a radial acceleration because its in circular motion as well as a tangential acceleration.