A disk with moment of inertia I1 rotates about a frictionless, vertical axle with angular speed wi . A second disk, this one having moment of inertia I2 and initially not rotating, drops onto the first disk (Fig. P11.30). Because of friction between the surfaces, the two eventually reach the same angular speed wf . (a) Calculate wf . (b) Calculate the ratio of the final to the initial rotational energy.