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Homework Help: Rotational Dynamics Help Needed

  1. Oct 20, 2008 #1
    I'm sure there is something ridiculously simple I'm missing here but this questions has been driving me crazy and now I'm sure I couldn't see the answer if it stared me right in the face.

    1. The problem statement, all variables and given/known data

    A massless string is wrapped around a solid cylinder. A block of mass m=2.0kg hangs from the string. When released the block falls a distance of 82 cm in 2.0s. Starting with a freebody diagram calculate the mass of the cylinder.

    *I can't use conservation of energy to solve this as the next question indicates that I am to solve the question again using conservation of mechanical energy.

    2. Relevant equations
    y = y0 + v0t + 1/2at2
    torsion = rF
    Net Torsion = Moment of Inertia x rotational acceleration

    3. The attempt at a solution

    r = ? **This is the part that has me stumped - all of the formulae I know need a radius.
    m = 2.0kg
    y = 0.82m
    t = 2.0s
    v0 = 0m/s
    mc = ?

    Step One: Determine the force of inertia of the cylinder:

    y = y0 + v0t + 1/2at2
    a = (2 x 0.82m) / (2.0s2)
    a = 0.41 m/s2
    Fa = ma = 0.41m/s2 x 2.0kg = 0.82N
    Fg = mg = 9.81m/s2 x 2.0kg = 19.62N
    FI = Fg - Fa = 19.62N - 0.82N = 18.8N

    This is as far as I can get before I start running into brick walls. I've played with several different formulae but I always run into a point where I need the radius of the cylinder (which I don't have). Am I missing something really obvious here?
  2. jcsd
  3. Oct 20, 2008 #2


    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper

    Try solving it anyway, just leave the radius as "R" without plugging in a number for it. See what you can come up with that way.

    p.s. Welcome to PF.
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