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Pendulum problem - What is the magnitude of the torque?

  1. Dec 11, 2005 #1
    a small mass of 0.75kg is attached to one end of a 1.25m long massless rod, and the other end is hung from a pivot. When the resulting pendulum is 30degrees from the vertical what is the magnitude of the torque about the pivot?

    what i did:
    -Fsin30 -mg = 0
    Fsin30 = -mg
    F = -14.7N

    t = rFsin30
    t = 1.25(-14.7sin30)
    t = -9.2N

    the answer is supose to be half of this what did i do wrong?
     
  2. jcsd
  3. Dec 11, 2005 #2
    I always always draw out the problem and figure it out geometrically on paper rather than resort to filling in formuals. Check out the attachment. Its easy to see from this that you need to find the vector mag. that acts on the tangent of the rods arc. Get the angle. Its 60. Get the resolved vector magnitude.

    cos 60= F/7.36

    Torque = 3.68 x 1.25
    =4.6 Nm

    Was your diagram maybe screwed up?>?
     

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