# Homework Help: Momentum Conservation of Turntable and Beetle System

1. Nov 11, 2009

### amb202

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

A beetle with a mass of 30.0 g is initially at rest on the outer edge of a horizontal turntable that is also initially at rest. The turntable, which is free to rotate with no friction about an axis through its center, has a mass of 80.0 g and can be treated as a uniform disk. The beetle then starts to walk around the edge of the turntable, traveling at an angular velocity of 0.0600 rad/s clockwise with respect to the turntable.

a. With respect to you, motionless as you watch the beetle and turntable, what is the angular velocity of the beetle? Use a positive sign if the answer is clockwise, and a negative sign if the answer is counter-clockwise.

b. What is the angular velocity of the turntable (with respect to you)? Use a positive sign if the answer is clockwise, and a negative sign if the answer is counter-clockwise.

c. If a mark is placed on the turntable at the beetle’s starting point, how long does it take the beetle to reach the mark again?

2. Relevant equations

L= I theta L= mvrsin

3. The attempt at a solution

I know in order to find the angular velocity of the beetle with respect to me, i need to know the angular velocity of the turntable, but without the radius, i cannot figure out how to find this.

2. Nov 11, 2009

### tiny-tim

Welcome to PF!

Hi amb202! Welcome to PF!
Just call the radius r … you'll find it drops out in the end.

3. Nov 11, 2009

### amb202

But wouldn't I need the angular velocity of the turntable to figure out the velocity of the beetle with respect to me?

4. Nov 12, 2009

### tiny-tim

Yes, and you can find that from the information given.

This is physics, not geometry …

you can't solve this just with geometry, you need a physical equation also …

which one do you think it is?