# Confused about the speed of a point on a rotating body

1. Nov 11, 2013

### kineticjustin

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

The system consists of a 20-lb disk A, 4lb slender rod BC, and a 1-lb smooth collar C. If the disk rolls without slipping, determine the velocity of the collar at the instant theta=30 degrees. The system is released from rest when theta= 45 degrees.

2. Relevant equations

v=wr

3. The attempt at a solution

2. Nov 11, 2013

### Simon Bridge

The radius is measured from the center of rotation.

But I suspect you have misunderstood the question - point C (the collar) is not rotating.

3. Nov 12, 2013

### kineticjustin

Would the center of rotation be C or G?

4. Nov 12, 2013

### Simon Bridge

The rod is certainly turning - but it's also translating (well, it's center of mass is).
It's rather like those problems with a ladder leaning against a wall and sliding isn't it?

How could you go about figuring the center of rotation - could it be B?
Why not draw a free-body diagram and place the forces on it and let them tell you?
(Your attachment, btw, is not a free body diagram.)