# Work done in Rotational Kinematics

1. Jan 14, 2010

### nahanksh

1. The problem statement, all variables and given/known data
A woman whose mass is 70 kg stands at the rim of a horizontal turntable that has a moment of inertia 300 kg m^2 and radius of 2 m. The system is initially at rest and is free to rotate about frictionless vertical axle through the center of the turntable. The woman begins to walk clockwise around the rim at a speed 1.5 m/s relative to the earth.

How much work does the woman do to set the system (woman plus turntable) in motion?

2. Relevant equations

3. The attempt at a solution
I used the given speed and got the kinematic energy of a woman as 0.5*m*v^2 = 78.75J.
Even if i am pretty sure there should be some other work done regarding the disk,
i can't figure it out which value i should substitute in...

P.S. when i used 0.5*(Idisk + Iwoman)*(final angular speed of the disk+woman), it was wrong..

Please could someone help me out here to get the rotational kinematic energy?

2. Jan 15, 2010

### tiny-tim

Hi nahanksh!

You also need the rotational KE of the disc, which is 1/2 Iω2.

(btw, you can only use (Idisk + Iwoman) if they have the same angular velocity )