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Homework Help: Double Rotating disks

  1. Mar 1, 2009 #1
    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?
  2. jcsd
  3. Mar 1, 2009 #2
    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?
  4. Mar 1, 2009 #3
    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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