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Homework Help: Torque and center of mass question

  1. Nov 28, 2005 #1
    We have a block of un-uniform density of mass m suspended by two massless wires, against gravity. The left wire makes an angle phi_1while the right angle makes an angle phi_2. The block has length L. They want me to find the center of mass.
    THis is what ive done:
    We have tension in the wires. Torque is force measured from a distance. In our case, the two wires create a torque about the center of mass. I find the forces,
    F_1 = sin(phi_1)*m*g
    F_2 = sin(phi_2)*m*g
    And realize that the torques are
    t_1 = F_1*x_c_m;
    t_2 = F_2*(L - x_c_m)
    since they equal another (net torque is zero), i set them t_1 = t_2 and solve for x. However the answer is incorrect and I am told to check my trignometry. My answer is
    x = (L sin(phi_2)) / (sin(phi_1)+sin(phi_2))

    whatever am i doing wrong?
  2. jcsd
  3. Nov 28, 2005 #2
    Where do you put your fulcrum?
  4. Nov 28, 2005 #3
    my fulcrum is x_cm, or the center of mass of the block from the left side.
  5. Nov 28, 2005 #4
    One question... are the wires attached to the ends of the block, you were not clear on that...
  6. Nov 28, 2005 #5
    yes they are. the wire making an angle phi_1 is on the left side and the wire making an angle phi_2 is on the right side.
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