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Homework Help: Angular Velocity w/2 welded disks

  1. May 15, 2013 #1
    1. The problem statement, all variables and given/known data

    Two concentric disks attached to each other form a single unit. This two disk unit is mounted onto a frictionless horizontal axle through the center hole. Two strings are attached and wound in opposite directions around the outer perimeter of each disk and are left hanging. The smaller disk has a mass of 1.66 kg and a radius R1 of 12.4 cm. The larger disk has a mass of 6.85 kg and a radius R2 of 24.8 cm. (a) If two identical masses of 800 g are hung from both strings and the two masses are released from rest from the same initial height what will be the angular velocity of the larger disk when the two masses are 60.0 cm apart? What will be the angular velocity of the smaller disk and the linear velocity of each mass at the same instant? (Figure 3)

    2. Relevant equations

    3. The attempt at a solution

    well I'll write down all the variables given

    *smaller* *larger* *mass1* *mass2*
    r1 = 12.4cm r2 = 24.8cm m1 = 800g m2 = 800g
    m1 = 1.66kg m2 = 6.85kg

    what we want is ω of larger disk, ω of smaller disk, and V_f of each mass

    Im not really sure where to get started here.

    Inertia of smaller disk = 1/2mR²
    Inertia of larger disk = 1/2mR²

    any hints here?
  2. jcsd
  3. May 15, 2013 #2

    Doc Al

    User Avatar

    Staff: Mentor

    You can either use dynamics (applying Newton's 2nd law to all three objects) or energy conservation.
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