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Homework Help: Torque on cantilever

  1. May 19, 2010 #1
    1. The problem statement, all variables and given/known data

    A mass of 50kg is hung from the end of a cantilever 3m long. The cantilever weighs 6kg and is supported by a cable from the wall that is attached 0.5m from the end of the cantilever forming a 30 degree angle with the cantilever.
    What is the tension in the wire?
    What is the vertical and horizontal components of the force exerted by the wall on the shelf?

    heres a picture i drew http://screencast.com/t/YjM4MmQ4M2Yt
    it says 30 degrees, 2.5m, 0.5m and 50kg in case u can't read it
    not pictured is the weight of the cantilever 0.6kg.

    2. Relevant equations
    torque = force * radius

    3. The attempt at a solution
    i'm very lost on this one.

    my guess that clockwise torque would be = 60 * 1.5 + 500* 3 = 1590 (what is the units for torque?)

    So this would mean that the ropes vertical component is 1590 right?
    this gives a tension of 3180 ( =1590/sin(30) )

    however the answers say the answer is 1300

    which i have no idea how
  2. jcsd
  3. May 19, 2010 #2


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    Science Advisor
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    Gold Member

    Torques are taken about a point..which point did you take? You are missing the torque from another force. Units for torque = force times length = newton-meters
    Why did you equate the vertical force to the torque? This is not correct. You might want to draw a free body diagram of the shelf and calculate the tension force by summing torques about the left support of the shelf (assuming a pinned support at that location capable of providing vertical and horizontal forces only). Note that the x and y components of the rope tension force are related by trig.
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