# Double Rotating disks

1. Mar 1, 2009

### Trentonx

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
M2
I = (.5)(.258)(.052) = .0003225 kg*m2
I found the angular velocity $$\omega$$
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?

2. Mar 1, 2009

### Dr.D

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?

3. Mar 1, 2009

### Trentonx

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.