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Homework Help: A couple questions regarding force and free body diagram

  1. Apr 23, 2014 #1
    Hey guy, I would like to ask a few question about force and free body diagram.

    First question, I conduct a compressive experiment a few day back, using compression machine to test the strength of different materials e.g., Aluminium, PVC and Ceramic. However there is one thing that I don't under stand in regards to this experiment. When the machine start working, the stress and strain graph somehow start up with a very shallow gradient instead of a straight linear gradient which one would expected for the elastic region (modulus of elasticity). This shallow gradient exists for all three experiments. I asked my lab adviser, which he hold me that this is normal. However, it still stuck me about where this shallow gradient come from.... can any one give me a clue for its existence.

    The second question has something to do with a question I found on a text book in regards to drawing a free body diagram. I'm a newbie at this and I'm not certain whether this is the right way to draw it. Can any one give me a head up. (The question and my work out is in attach files)

    Best Regards

    Attached Files:

  2. jcsd
  3. Apr 23, 2014 #2
    1) I assume that depending on how you attached the compressive element to the testing machine there was some initial slack to be taken up before the full compression load could be applied.

    2) Your free body diagram for AB is not in equilibrium. The FBD for BC is looks right.
  4. Apr 23, 2014 #3
    You should put it in the homework section of the forum. You will get better responses.
  5. Apr 24, 2014 #4
    Ah, I see that make sense why the graph start out shallow.

    For question 2, I thought the one that has a mistake in is segment 2. Is there any other force/moment involves in segment 1? I thought the only force acting on it will be Fcl and Fbc, could there be a bending moment caused by Fcl in segnment A?
  6. Apr 24, 2014 #5
    In order for segment 1 to be in equilibrium you have to sum all the forces in both directions to zero and to sum the moments to zero. So you are missing something.
  7. Apr 28, 2014 #6


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    Staff: Mentor

    Moved. :smile:
  8. Apr 29, 2014 #7
    Berkeman, Kudos to keeping this forum neat :)!
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