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Find the tension in each rope torque

  1. Oct 24, 2007 #1
    Find the tension in each rope "torque"

    1. A uniform plank of length 2m and mass 30kg is supported by three ropes. Find the tension in each rope when 700N person is 0.5m from the left end.

    2. diagram
    ↑ /
    | /
    | /40 degrees ↓ (rotating clockwise)
    ←-----person-----------------/---


    3. The attempt at a solution

    So i began with isolating the X and Y variables
    ie: X on left, fcos40
    y going up, fsin40
    how would i go about breaking down the mass of the person + plank to figure out the tension in each rope?
     
  2. jcsd
  3. Oct 24, 2007 #2

    Astronuc

    User Avatar

    Staff: Mentor

    For a statics problem the sum of the moments about any pivot is zero. It's a little difficult to tell from the illustration where the ropes are, but I'm guessing all three are attached at the left end?

    Treat the person as a point load from at 0.5 m from the end, or 1.5 m from the pivot. Treat the weight of the plank at its center of mass.
     
  4. Oct 24, 2007 #3
    sorry the diagram didn't show up better

    theres a rope pointing left, one pointing up (on the left side) and one rope pointing tot he right at 40 degrees North east
     
  5. Oct 24, 2007 #4

    Astronuc

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    Staff: Mentor

    And the pivot is on the right?

    If there is no tension in the plank then then the x- or horizontal component in the rope at 40° will balance the tension in the horizontal rope.

    Try to write the equation for the sum of the moments about the pivot.
     
  6. Oct 26, 2007 #5
    awsome, figured it out.
    i set x coord t1cosO - T3 and y coord as T1sinO + T2 - W(person+plank)

    set my t1 = 1/4(Wperson +2Mplank)/sinO then i just plugged in T1 to my other forumlas.

    thanks!
     
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