a disk with moment of inertia i1 rotates about a fric- tionless, vertical axle with angular speed vi . 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 even- tually reach the same angular speed vf . (a) calculate vf . (b) calculate the ratio of the final to the initial rotational energy.