While redesigning a rocket engine, you want to reduce its weight by replacing a solid spherical part with a hollow spherical shell of the same size. The parts rotate about an axis through their center. You need to make sure that the new part always has the same rotational kinetic energy as the original part had at any given rate of rotation. If the original part had mass ?M?, what must be the mass of the new part?

Solution 76P Step 1: The kinetic energy of rotating disk is K.E = 1/2 Iw 2 To have same kinetic energy for any as the two parts.must have same moment of inertia.