Another rotational motion problem.
An overhead view of a ring that can rotate about its center like a merry go round. Its outer radius R is 0.8 m, its inner radius r is 0.4 m, its mass is 8 kg, and the mass of the crossbars in the ring is negligible. It initially rotates at an angular speed of 8rad/s with a cat of mass 2 kg on its outer edge, at radius R. By how much does the cat increase kinetic energy of the catring system if the cat crawls to the inner edge, at radius r? (The answer is 39.1J  From textbook)
I don't know the solution.
I tried to find initial and final moment of inertia (I)
Iinitial = (0.5)(8)((0.8^2)+(0.4^2)) + 2(0.8^2) = 4.48 kgm^2
Ifinal = (0.5)(8)((0.8^2)+(0.4^2)) + 2(0.4^2) = 3.52 kgm^2
Then I don't know how to do it.
Please help.
Thank you.
