Disk and cylindrical shell collision using angular momentum conservation

[Jim011991]

Homework Statement

A uniform disk of mass 2kg and radius 10.0cm is spinning on a fixed frictionless bearing at 3 revolutions/second. A think cylindrical shell of mass 2kg and radius 10cm is carefully dropped onto the spinning disk so that they end up spinning together, with their edges coinciding.

a) How fast (in revolutions/second) are they spinning after the drop?
b) Is kinetic energy conserved is this process? If not, by what percentage has it changed?

Homework Equations

m*w*r^2

The Attempt at a Solution

a) 2*3*10^2 = 600
600 = 4*w*10^2
600 = 400w
w=1.5 rev/sec

b) Kinetic energy is conserved in this process.

Did I do this correctly?

 

[Quinzio]

Point a)
In the end the result is correct, but it's wrong the way you get it.
You use wrong units, but in the end they cancel out each other, so you get the good result.

Point b)
No, it's wrong.