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Homework Help: Beam deflection

  1. Jul 21, 2016 #1
    1. The problem statement, all variables and given/known data
    for the circled part, why the why the author didnt include -12(9)kNm moment in the equation of moment ?
    [PLAIN]http://imgur.com/a/l3Bu4[URL]http://imgur.com/a/l3Bu4[/URL] [Broken]
    2. Relevant equations

    3. The attempt at a solution
    IMO, the author should include it in the equation, am i right?
    So, M= -258+52<x-0>-(0.5)(8)(<x-0>^2)+50+(0.5)(8)(<x-5>^2)-12(9) ?
    Last edited by a moderator: May 8, 2017
  2. jcsd
  3. Jul 21, 2016 #2

    Attached Files:

    • 312.jpg
      File size:
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  4. Jul 22, 2016 #3
    did the author miss out something?
  5. Jul 27, 2016 #4
    The figure does not include the variable 'x'. But, wherever you put it in the range 0 to 9 m, what contribution to Mx do you think the 12 kN force would make? Recall one definition of moment that it is the algebraic sum of moments on ONE side of a section.
  6. Jul 27, 2016 #5
    The force 12kN only will cause moment at x just 9m, no moment generated at x less than 9m....
    But, I still couldn't understand why the moment 12(9) is not included?
  7. Jul 27, 2016 #6
    do you mean the x value range from 0 to 9 ? it cant be more than 9 , because there's no moment beyond 9m ?
  8. Jul 27, 2016 #7
    Because the expression for M is the summation of moments to the LEFT of section X, but the 12 kN load is to the right. If you were to write down an expression for the moment to the RIGHT of x, you would get an expression that may at first sight look different, but, if correctly drawn up, will be exactly equivalent (and incidentally acts as a check on whether you have made any mistakes of the kind you are making now). I suggest you try that to reinforce the correct application of the definition of bending moment.
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