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Homework Help: Angular Momentum and Torque

  1. Jun 28, 2011 #1
    1. The problem statement, all variables and given/known data

    I was just doing some extra credit problems and couldn't figure this one out.
    I'd appreciate any help! Thanks!

    A mass of 5.6 kg is attached to a light cord, which is wound around a pulley. The pulley is a uniform solid cylinder of radius 7.3 cm and mass 2.7 kg.

    1 - What is the magnitude of the net torque on the system about the point on the axis at the center of the pulley? ( in N m)

    2 - When the mass has a speed of v = 49 m/s, the pulley has an angular speed of w = v/R
    Determine the magnitude of the total angular momentum of the system about a point on the axis at the center of the pulley (kg m^2/s)

    3 - using the fact that avg torque = dL/ (dt) and your result from the previous section, calculate the magnitude of the acceleration of the mass. (m/s^2)

    2. Relevant equations

    3. The attempt at a solution

    I tried using torque = inertia x angular acceleration
    I also tried
    F = mg - ma = 5.6*9.81 - 5.6*a
    so τ = F*R = (mg - ma)*R
    I = 1/2*mR^2 = 1/2*2.7*0..073^2 = 0.007194 kgm^2 but got stuck and this didn't seem right.

    I am just really confused. Thanks for all your help
  2. jcsd
  3. Jun 28, 2011 #2

    Doc Al

    User Avatar

    Staff: Mentor

    What forces act on the system? What torque do they exert about the given axis?
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