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Finding net torque

  1. Apr 30, 2009 #1
    1. The problem statement, all variables and given/known data
    A cylinder having a mass of 2.0kg can rotate about its central axis through point O. Forces are applied as show: F1 = 6.0N, F2 = 4.0N, F3 = 2.0 N, and F4 = 5.0N. Also, r = 5.0cm and R = 12 cm. Find the (a) magnitude and (b) direction of the angular acceleration of the cylinder.

    2. Relevant equations

    3. The attempt at a solution
    as of yet, i'm not looking for the solution to the entire problem, because i'm stuck on the first step of finding the net torque. this is what i have:

    net torque = F1R - F2R - F3r + F4R = 6*0.12 - 4*0.12 - 2*0.05 + 0 = 0.14 N m

    the book's solution for the net torque is 71 N m. i also got a similar problem wrong for presumably the same reason. the units are newtons and meters, and i'm sure my calculator works fine. what is going on here?
  2. jcsd
  3. Apr 30, 2009 #2


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    Homework Helper

    torque = I*alpha.
    Find the moment of inertia of the cylinder and then find angular acceleration.
  4. Apr 30, 2009 #3
    thanks... but can you please tell me why what i was doing was wrong?
  5. Apr 30, 2009 #4
    F4 is considered as torque with respect to the center?
    oh just saw that You wrote 0, my bad.
    Whati s the answer in the book for the angular acc?
  6. May 1, 2009 #5
    i'd like to go back to this step, because this is what i found and this is what was written in the student solutions manual, EXCEPT the answer turned out to be 71 N m instead of 0.14 N m and i can't see how that could possibly be the case. i'm also getting other problems wrong at this step, and i can't move forward until i see what i'm doing wrong here, so if anyone could explain to me this particular step, instead of how to solve the entire thing, that would be a really big help to me. thanks!
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