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Homework Help: Torque pulley problem

  1. Apr 28, 2010 #1
    1. The problem statement, all variables and given/known data
    Untitled.jpg
    the problem statement is in the pic.



    2. Relevant equations
    inertia for cylinder shape= m*r^2*1/2
    torque=F*"length
    sum of torque= "inertia" * "angular acceleration"
    w=mg



    3. The attempt at a solution
    i was thinking that the weight forces of the masses times the length would give me the individual torques. i converted the cm to m to stay in SI units. having the counterclockwise as my positive direction i set up my equation:
    (9.8m/s^2*1kg*.1m) + (9.8m/s^2*4.9kg*.1m)= 1/2*4kg*.1m^2* "angular acceleration".
    i got "angular acceleration" = 191.1 rad/s^2. i entered this into the computer and i confirmed that it was wrong.

    i was thinking that maybe i needed to use the 2 different tension forces in place of the gravitational forces that i used in my torque equation. but in that case i end up with an equation with 3 unknowns. so i'm not sure what to do?
     
  2. jcsd
  3. Apr 28, 2010 #2
    wouldn't you need the moment of inertia for the pulley to?
     
  4. Apr 28, 2010 #3
    in the problem i have enough info to calculate the moment of inertia:

    1/2 *m *r^2 using the given mass and radius of the pulley.

    1/2 *4kg*.1^2= .02 kg m
     
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