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Homework Help: Method of Sections

  1. Jun 16, 2012 #1


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    Hi guys im having trouble taking the moment after i cut the truss, the answear always seems to be off. I attached my work in the attachements. Can you check if my equations are right.

    for moment: Mom@c=0: (ACx2.2)+(18.72x2.2)+(24x2.2)=0, AC=42.7??
    sum of all forces in x direction: Fx=0: -24+AC+BD+BCsin(30)=0
    sum of all forces in y direction: Fy=0: -18.72+BCcos(30)=0

    ****One of my main problems is the cos and sine values because some times x is cos and sometimes its sine when you doing trusses is there a trick on how i can tell which one to use?****

    Attached Files:

    Last edited: Jun 16, 2012
  2. jcsd
  3. Jun 22, 2012 #2
    In the method of sections you should draw a diagram to show where you have taken a section. In your first eqn, you start with the statement Mom@c=0 but go on to say AC x 2.2 etc But Force AC passes through c. So, taking moments about C should result in a zero moment contribution from the force AC.. The rest of your equation contains forces to the left and tot the right of C. This underlines the importance of sketching where the section is that you are considering and isolating that part of the frame (to the left or to the right of your section) about whose equilibrium you intend to make a statement. We could deal with sin and cos after you have started correctly.
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