1. The problem statement, all variables and given/known data A hollow cylinder has mass m, an outside radius R2, and an inside radius R1. Use integration to show that the moment of inertia about its axis is given by I = 1/2*m(R2^2 + R1^2) 2. Relevant equations dm = rho*dV = 2*pi*rho*h*r*dr 3. The attempt at a solution This doesn't really concern the solution of the problem. There's something else that's bugging me. If, ultimately, the solution and the moment of inertia itself in this case doesn't depend on h (because the mass is distributed evenly along h?), why the need to express an infinitesimal element, dm, of the body by using the volume? We know that the moment of inertia for a solid cylinder is the same as that of a thin circular plate. And so, in finding the moment of inertia of a solid cylinder, I = 1/2*MR^2, one doesn't have to concern oneself with its height. I guess my question then is, why one cannot express an infinitesimal element of the hollow cylinder by using area instead of volume?