1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Angular Acceleration of a Pulley Supporting Two Hanging Masses

  1. Apr 28, 2012 #1
    1. The problem statement, all variables and given/known data

    Hi all! I'm working on a dynamics homework and have hit a wall here:


    "If the frictional moment at the pivot O is 2 N m, determine the angular acceleration of the grooved drum, which has a mass of 8 kg and a radius of gyration k = 225 mm.

    Ans: α = 0.622 rad/s^2 "

    2. Relevant equations

    M = m g r

    ƩM = I α

    I = m k^2

    3. The attempt at a solution

    The two masses exert moments about O, so my first step is to sum the moments about the pivot O:

    ƩM = (12 kg)(9.81 m/s^2)(0.2 m) - (7 kg)(9.81 m/s^2)(0.3 m)
    ƩM = 2.943 N m

    These moments are resisted by the frictional moment such that:

    ƩM = (2.943 N m) - (2 N m)
    ƩM = 0.943 N m

    Next we need the moment of inertia of the pulley:

    I = m_pulley * k^2
    I = (8 kg)(0.225 m)^2
    I = 0.405 kg m^2

    Finally we can solve for angular acceleration:

    ƩM = I α
    α = ƩM / I
    α = (0.943 N m) / (0.405 kg m^2)
    α = 2.33 1 / s^2

    My analysis seems reasonable and I come up with the right units, but it is not very close to the given answer.

    Am I doing something wrong?
    Last edited: Apr 28, 2012
  2. jcsd
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Can you offer guidance or do you also need help?
Draft saved Draft deleted