Conservation of angular momentum | Rotating disc with moving cockroach

A cockroach of mass m = 5.5 g lies on the rim of a uniform disk of mass 22 g that can rotate freely about its center like a merry-go-round. Initially the cockroach and disk rotate together with an angular velocity of 0.260 rad/s. Then the cockroach walks halfway to the center of the disk. (a) What then is the angular velocity of the cockroach-disk system?
(b) What is the initial (Ki) and final (Kf) kinetic energy of the system?
(c) What accounts for the change in the kinetic energy?
Solution is based on the application of conservation angular momentum law which is valid because the net torque on the disc-cockroach system is equal to zero. The change of kinetic energy of the system is due to existence of nonconservative internal force which pushes the cockroach along the disk. This force originates from the transformation of chemical energy stored in cockroach, and leads to change of mechanical energy of the system
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