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Homework Help: Solid mechanics-how do i find the maximum momentum in this question?

  1. Nov 7, 2009 #1
    Solid mechanics-how do i find the maximum momentum in this question?
    in this question, i am given the function for the xx stress as seen in the diagram below:
    http://lh6.ggpht.com/_H4Iz7SmBrbk/SvG3wUaPVAI/AAAAAAAAB6w/sMKXVGWomi8/Capture1.JPG [Broken]

    i am asked to sketch the stress distribution, which i have done
    http://lh3.ggpht.com/_H4Iz7SmBrbk/SvG3wPYlmdI/AAAAAAAAB6s/rd9T_ED6MK0/Capture.JPG [Broken]

    now i am asked to find the value of C which will give me the maximum value for Mo, which as far as i can see is Moment about the z axis.

    what ive done is

    Mz= -ʃʃ {(σxx)*y} dA

    now i know that there are 3 options,(3 functions for σxx) but i know that for y>=c and y<c the function for σxx is constant and -ʃʃ {y} dA is 0,(1st moment of area)

    therefore i think i must find it where -c<y<c,

    -ʃʃ {(-σy/c)*y} dA= σ/c * ʃʃ {(y^2} dA
    =σ/c * ʃʃ {(y^2} dydz [(z from -b/2 to b/2), (y from -h/2 to h/2)]
    and i get


    now how do i find thevalue for c that will give me the maximum Mo?? if c=0 Mo is infinite ??

    i thought maybe to find the derivative and compare to 0 but by what? y? z? either way i dont see how that would help, since i am looking for the maximum "c" value, then i thought maybe dMo/dC but i dont think so.
    Last edited by a moderator: May 4, 2017
  2. jcsd
  3. Nov 7, 2009 #2
    never mind, my integral was totally wrong, got it right though, c=0, thanks anyway
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