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Homework Help: Method O' Joints, question about the Method itself, semi- !

  1. Dec 28, 2011 #1
    Method O' Joints, question about the Method itself, semi-urgent!

    1. The problem statement, all variables and given/known data

    Hi everyone, my first post! :D This is more of a general question about the method rather than a specific example.

    I have a problem with substitution while I'm doing the method of joints for a loaded truss thing; for example, I get this as my answer for one the forces on one of the beams:

    Force (from point A to point B) = -10000 N

    Assumptions/Sign Convention, etc.:
    I originally assumed all forces are going away from the joint (in this case, point A).
    I considered all forces going away from point A as tensile (positive, +).
    I considered all forces going towards point A as compressive (negative, -).
    So, since I got -10000 N, it is compressive (-) and not tensile as I originally assumed?

    Now I have, for example, another equation which wants me to substitute this value into it, to find the force on another beam of the truss.

    My main question is, when substituting this force in, do I include the minus (-) sign in, so therefore I sub in -10000,

    or do I disregard the minus sign's existence, and sub in +10000?

    Does the minus (-) just represent the compressive nature of the force, or does it have an effect on my equation/subbing/thingy?

    2. Relevant equations
    3. The attempt at a solution

    I'm not sure which way to go?...
  2. jcsd
  3. Dec 28, 2011 #2


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    Re: Method O' Joints, question about the Method itself, semi-urgent!

    Hello blargonaut. Welcome to PF !

    To answer your first question, Follow your sign convention. If you are using negative for compressive force, and the answer is negative, then the force on that beam is compressive.

    If the force is compressive at one end of the beam, then it must be compressive at the other end.
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