Disk a, with a mass of 2.0 kg and a radius of 40 cm, rotates clockwise about a frictionless vertical axle at 30 rev/s. disk b, also 2.0 kg but with a radius of 20 cm, rotates counterclock- wise about that same axle, but at a greater height than disk a, at 30 rev/s. disk b slides down the axle until it lands on top of disk a, after which they rotate together. after the collision, what is their common angular speed (in rev/s) and in which direction do they rotate?