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Homework Help: Help understanding Truss answer

  1. Nov 27, 2013 #1
    1. The problem statement, all variables and given/known data

    The truss is suspended by two pin joints. Each segment is 3 m wide and 4 m high. The applied force is F = 1 N. What is the force in member 2?


    2. Relevant equations

    Sum of force = 0

    3. The attempt at a solution

    Using method of sections (to get force at 3), so taking the right portion

    I am trying to understand the sum of the force in the y direction of the section.

    sum of force y of portion =0 =-1 - J_3 cos(180-arctan(3/4))

    My question, do we use 180-angle when looking at the angle,

    also is it negative J_3, since the J_3 is pointing downwards?

    I already know that the answer is 1N, I just don't understand this part.

    Thank you.
  2. jcsd
  3. Nov 28, 2013 #2
    Any ideas?
  4. Nov 28, 2013 #3


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    Science Advisor
    Homework Helper
    Gold Member

    I think you are looking for force in 3 by cutting right hand section thru 12,3, and 9? Forget about signs plus or minus from trig. You know that F acts down, so the vert force in 3 must act up, pushing in towards the member. Pushing forces are compression forces, usually designate as negative. Tension forces are positive. Use the properties of a 3-4-5 right triangle to get the compressive force in member 3.
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