Question

A CD player rotates at a variable speed so that a laser can scan pits and land on the disk's bottom surface at a constant tangential speed of 1.2 m/s. The disk has a moment of inertia of 1.2x10^-4 kg m^2 and the music is first detected when the laser is located 15 mm from the disk's center. Assuming the disk starts from rest, find the work done by the motor during this start-up.

Answer 1

Work done by the motor goes to rotational energy of the disk.
you need to calculate:
\[W=K_f-K_i=K_f=\frac{I\omega^2}{2}\]
since it starts from zero.
and \[\omega=v/r\]
you have I, v, r.