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Homework Help: Torque of a bar on a crane

  1. Feb 25, 2013 #1
    1. The problem statement, all variables and given/known data

    You are a member of a medical forensics team
    investigating a fatal accident at a freight
    yard. A new crane for lifting cargo broke
    while lifting a package and the police suspect
    sabotage. The crane consists of 45.0 kg, 15 ft
    lightweight aluminum bar supported at its
    base by a hinge that allows the bar to pivot
    vertically. A support cable runs from the
    other end of the bar to the ground. When the
    hinge failed, the bar was at an angle of 45° to
    the ground and the support cable was at an
    angle of 30° to the ground. The yard owner
    says it is impossible for this to be an accident
    since the hinge is rated to support a force of
    three times the weight of the 225 kg package
    that was being lifted. The package was lifted
    straight up from the end of the bar at a
    constant speed.

    2. Relevant equations

    Torque = Force x Length

    Weight = Mass x Gravity

    ƩFx = 0

    ƩFy = 0

    ƩT = 0

    T = Torque

    3. The attempt at a solution

    I'm having trouble setting up the picture for this problem. I understand how to solve for forces and am still getting comfortable with torque but I'm having a difficult time drawing out the picture for this problem as I felt like the information given was a bit vague. Any help is appreciated!
  2. jcsd
  3. Feb 25, 2013 #2


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    Science Advisor
    Homework Helper
    Gold Member

    Draw a 45 degree right angled triangle, with the horizontal base, AB and the vertical, AC, being the two equal sides. The hypotenuse, BC, represents the bar, and the vertical is the load cable.
    Extend the base AB out beyond B to a point P s.t. angle APC is 30 degrees. PC is the support cable.
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