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Statics free body diagram concept question

  1. May 30, 2012 #1
    1. The problem statement, all variables and given/known data
    See attachments


    2. Relevant equations



    3. The attempt at a solution
    In the explanation attachment, I don't get the last paragraph "Note that force Fce acts downward on bar....compression."

    Since from the figure Fce is pointing down, and Fbd is pointing up, shouldn't bar BD be in tension and bar CE in compression? I know intuitively just by looking at the figure, yes, obviously BD is in compression and CE is in tension, but I'm confused by the free body diagram the book draws and its subsequent explanation. I must be missing some concept here. Thanks.
     

    Attached Files:

  2. jcsd
  3. May 30, 2012 #2

    hotvette

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    The horizontal member pushes down on member BD, placing it in compression. In turn, member BD pushes up on the horizontal member (equal and opposite). Thus, FBD points up in the bottom picture, which is a free body diagram of the horizontal member. A free body diagram of member BD will have an force pointing down at the top and a force pointing up at the bottom (ground reaction). That's how free body diagrams work.
     
    Last edited: May 30, 2012
  4. May 30, 2012 #3
    ah so the figure is a free body diagram of the *bar ABC* and not member BD, so it only displays the forces relative to how they effect ABC? Thanks a lot.
     
  5. May 30, 2012 #4

    hotvette

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    Correct.
     
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