# Find angular velocity using angular momentum

A 20kg girl stands on a large turntable 4m from its center. The turntable has a moment of inertia (about its roation axis) of 200kgm^2. Assume that the girl is a point mass.

a) Assume that the girl and turntable are rotating at 1 rad/s. If the girl walks inwards to a radius of 3m and stops, what is the new angular velocity?

b) Find the change in kinetic energy.

I got these two parts, but I am stuck with c and d

c) Assume that the turnable is initially stationary and the girl is at r=4m.

The girl starts walking in a circle around the turntable at a speed of 2m/s relative to the ground. Find the angular velocity of the turntable relative to the ground.

d) The girl starts walking in a circle around the turntable at a speed of 2m/sy relative to the turntable. Find the angular velocity of the turntable relative to the ground.

c) I got from part a that w_i = 1 rad/s and w_f 1.37 rad/s. I know that that I = 200 + mr^2

However, I have no idea what to do when she walks 2m/s. As she gets closer to the center, the turntable will spin faster. But I don't know what to do for c and d.

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