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Multiple torques/forces on a disk, finding net torque

  1. Apr 29, 2007 #1
    1. The problem statement, all variables and given/known data

    The 20-cm-diameter disk in the figure can rotate on an axle through its center.

    What is the net torque about the axle?

    image: http://session.masteringphysics.com/problemAsset/1000963/5/knight_Figure_13_14.jpg

    2. Relevant equations

    τ=Frsinφ

    3. The attempt at a solution

    τnet= τ1+τ2+τ3+τ4

    τ1 = 30N * 0.1 * sin(90)
    τ2 = 30N * 0.05 * sin(-45)
    τ3 = 20N * 0.05 * sin(-90)
    τ4 = 20N * 0.1 * sin(45)
    _____________________
    τnet =~2.35355 Nm

    this was my original solution, which came out incorrect, a friend then told me that τ4 is zero because the force of 20N is not being applied to a point on the disk itself, so then:

    τ1 = 30N * 0.1 * sin(90)
    τ2 = 30N * 0.05 * sin(-45)
    τ3 = 20N * 0.05 * sin(-90)
    _____________________
    τnet =~0.94 Nm

    the issue is that I have only one attempt left at this problem and I want to confirm this before I submit the problem
    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
     
  2. jcsd
  3. Apr 30, 2007 #2
    That looks kike it should be the right answer, but I think it is less confusing and error prone if you keep the signs out of the sin arguments, in other words if a force is tending to cause conterclockwise rotation, make the entire torgue negative,

    eg, T2=-30N*.05m*sin(45)
    T3=-20N*0.05*sin(90)

    Else it looks fine.
     
  4. May 1, 2007 #3
    got it, yea the signs were messing me up, i got 0.94 when its actually -0.94,

    thanks
     
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