Common angular speed after collision

[csimon863]

Homework Statement

Disk A, with a mass of 9.0 and a radius of 90 , rotates clockwise about a frictionless vertical axle at 30 . Disk B, also 9.0 but with a radius of 10 , rotates counterclockwise about that same axle, but at a greater height than disk A, at 30 . Disk B slides down the axle until it lands on top of disk A, after which they rotate together.

Homework Equations

ive been trying to find the acceleration? a=delta w/delta t but obviously this isn't wokring!

The Attempt at a Solution

I can't even attempt a solution. I am completely lost on this one, so much so i registered for physics forum just to get some help.

 

[denverdoc]

Whats the question? Edit: I see it now in the heading. What about conservation of angular momenta?

 

[csimon863]

the setup: disk a has a mass of 9.0 kg and a radius of 90cm, rotates clockwise about a frictionless vertical axle at 30 rev/s. disk b, also 9.0 kg but with a radius of 10cm rotates counterclockwise 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?

i tried:
A has L= I x w, I = mass x rad^2 = 7.29 x -30 = -218.7 L for b = +2.7. So the final L = -216 CW

its not working. very frustrating.

 

[denverdoc]

maybe the math--using cm-kg: this is the (1/2 mR^2) x revs/sec
a: +9 x 8100/2 x 30
b: -9 x 100/2 x 30
Sum: 9 x 8000/2 x 30 = 36000 x 30 = 108 x 10^4

Sum for the composite= [9 x 8100/2 + 9 x 100/2] x ? =9 x 4100 x ?
? = 29.27 RPS

PS: clockwise rotation I define as positive quantity.