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Homework Help: Square metal plate torque

  1. Nov 20, 2007 #1

    A square metal plate 0.180 meters on each side is pivoted about an axis through point O at its center and perpendicular to the plate.

    Calculate the net torque about this axis due to the three forces shown in the figure if the magnitudes of the forces are F1 = 18.0 N F2 = 27.0 N and F3 = 14.0 N. The plate and all forces are in the plane of the page.

    Torque = Force x Length
    Length = 1/2 x 0.180
    Torque = radius x F sin theta

    T1 (1.62) - T2 (2.43) - T3 (1.26) = -2.07

    Shouldn't T1 be positive since it is going in the CW direction, while T2 and T3 are negative since they are going in the CCW direction? Where am I going wrong?
  2. jcsd
  3. Nov 20, 2007 #2

    Doc Al

    User Avatar

    Staff: Mentor

    The usual sign convention is to take CCW torques to be positive and CW torques to be negative. More important that sign convention: Recalculate the torque due to force F3.
  4. Nov 20, 2007 #3
    Thanks. After 15 mins of struggling, I finally figured it out where I went wrong with T3. :)
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