This question really has me stumped. Am I right to just ignore the fact that there is an inner control room ring? This is what I think I will need: We haven't gotten to inertia yet, but I think this is an inertia problem. I = MR^2 (if I take out the inner ring and say it is a thin-walled hollow cylinder). K = (1/2)*I*(omega)^2 I am not sure what I am suppose to do with the density of the core without a volume or mass. I am also not quite sure where I would bring in value of earth's gravity to solve with. Am I missing something to be able to start this?