Double Rotating disks

1. The problem statement, all variables and given/known data
A disk of mass M1 = 350 g and radius R1 = 10 cm rotates about its symmetry axis at f_initial = 152 rpm. A second disk of mass M2 = 258 g and radius R2 = 5 cm, initially not rotating, is dropped on top of the first. Frictional forces act to bring the two disks to a common rotational speed f_final.
a) What is f_final? Please give your answer in units of rpm, but do not enter the units

2. Relevant equations
T = I*a
I = (m*r2)/2

3. The attempt at a solution
I found the moment of inertia of M1
I = (.5)(.35)(.12) = .00175 kg*m2
I = (.5)(.258)(.052) = .0003225 kg*m2
I found the angular velocity [tex]\omega[/tex]
152rpm = 304pi rads/min = 5.06pi rads/sec
I don't suppose that I can just consider the mass to have just increased can I, because it specifies friction. I know some numbers, but how to put them together?
You know that energy is not conserved because friction is acting and heat is being generated. However, something else is being conserved. What is it?
L = I*w and Li = Lf
Momentum would be conserved because the net torques act internally to the system. I get it, thanks for the hint.

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