Posted by **Trace** on Wednesday, January 19, 2011 at 7:48pm.

this is the question :

A CD has a diameter of 12.0 cm and a mass of 15.8 g. The motor in a CD player causes the disk, initially at rest, to spin. When in operation, the CD spins in such a way that the laser scans the tracks at a constant tangential velocity of 1.20 m/s. The first track of the CD is at a radius of 20.0 mm from the centre of the disk. Calculate the work done by the motor in the CD player in bringing the disk from rest to the proper speed to read the first track. You may ignore the small circular hole at the CDâ€™s centre. [Hint: Recall the work-kinetic energy theorem.]

I got an angular speed of 60 rad/s, and now I'm trying to figure out the moment of inertia (I) so I can use the formula K rotational = 1/2 I w^2, then I should be able to equate work with kinetic energy using the formula K final = K initial + work, where K initial is zero.

but I don't know how to find moment of inertia. Do I just use the total mass of the cd given?