Flywheel rotates without friction at an angular velocity $0=600$ rev$/$min on a

frictionless, vertical shaft of negligible rotational inertia. A second flywheel,

which is at rest and has a moment of inertia three times that of the rotating

flywheel, is dropped onto it$.$ Because friction exists between the surfaces, the

flywheels very quickly reach the same rotational velocity, after which they

spin together