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Homework Help: Moment at 2 ends of beam

  1. Jul 6, 2016 #1
    1. The problem statement, all variables and given/known data
    in the first diagram , i notice that the moment is constant throughout the beam...

    why in the second notes, for the moment 20kNm at 2 ends of beam, why the moment varies from -20kNm to 0 from one end to another end?


    2. Relevant equations

    3. The attempt at a solution
    Which is correct? Which is wrong?
    i'm confused....
    another thing that i noticed is why no matter moment is clockwise or anticlockwise, the moment is always -20kNm??(as we can see ,moment is anticlockwise on the left , clockwise on the right....)
  2. jcsd
  3. Jul 6, 2016 #2


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    You can't compare beams willy-nilly. The support conditions influence how the forces and moments distribute within the beam.

    In the first case, you have a cantilever beam, with the left end fixed and a couple applied at the opposite end. In order to maintain static equilibrium, there will be a constant moment present along the length of the beam, as shown in Fig. (b). The fixed end of the cantilever has a reactive couple, which combines with the applied couple to make a zero net moment for the beam.

    In the other two beam cases, the beam is pinned at each end and a couple of magnitude 20 kN-m is also applied at each end. Pinned connections cannot support a moment, so the moment diagram shows the magnitude of each couple where it is applied and a zero moment at the opposite end of the beam, with a linear slope in between.
  4. Jul 6, 2016 #3
    why moment at the opposite end of the beam will be 0 ??? why not positive 20kNm ???
  5. Jul 6, 2016 #4


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    I explained that in my post.

    If you are not going to read the replies to your questions, what are you doing on this site?
  6. Jul 6, 2016 #5
    why not positive 20kNm ??? why it's 0 ? we can see , the moment 20kNm at the left ( anticlockwise) is opposite to the moment on the right (clockwise)
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