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Homework Help: Solving a system of equations

  1. Nov 10, 2007 #1
    1. The problem statement, all variables and given/known data

    The question is asking for the internal forces at E, but I get stuck solving the external reactions. After that, I can do it, and have, using electronically generated results. I just get stuck solving the system of equations I get after doing the determinant equations.

    We ignore the reactions at E. Those will be applied in the next step, where I find the internal forces there. I just drew them in to help me remember where they are, and to visualize the problem.

    The moment occurring between O and C has a magnitude of 300 kNm, and acts in the k direction.


    2. Relevant equations

    [tex] \Sigma M_x = A_z + 0.4B_z - 0.75C_y - 5 = 0[/tex]
    [tex] \Sigma M_y = A_z + 0.75C_x = 0[/tex]
    [tex] \Sigma M_z = -A_y - 0.4B_x - 10 = 0[/tex]
    [tex] \Sigma F_x = C_x + B_x + +7 = 0[/tex]
    [tex] \Sigma F_y = C_y + A_y - 12 = 0[/tex]
    [tex] \Sigma F_z = B_z + A_z - 5 = 0[/tex]

    3. The attempt at a solution

    I tried it for two different unknowns, and both times it didn't work. Sorry for the poor images; I did the work in pencil, and it wouldn't scan very well.



    Here are the real answers, and when I put my equations into a computer, it solves them and gets these. As such, I know my equations are correct. My problem just pops up when I try and solve for each unknown. If anyone could help me see where I went wrong, I'd be very grateful to them.

    Last edited: Nov 11, 2007
  2. jcsd
  3. Nov 10, 2007 #2
    So no one's able to help with this? Its driving me crazy; I've done the work on this four or five times now, and have gotten everything else I need for the question done already.

    Strongly tempted to just say that I used a computer to solve the system, as that isn't even the main point of the problem .
  4. Nov 11, 2007 #3

    Shooting Star

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    Perhaps the reason that nobody has been able to help was that there were no question anywhere.
  5. Nov 11, 2007 #4
    Um, I'd have thought I made it clear that I would like someone to check my work, to see if they could spot any errors. I wasn't just posting this for fun.

    Some reading comprehension skills may benefit you.
  6. Nov 11, 2007 #5

    Shooting Star

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    Thankfully, my comprehension skills do not extend to reading nonsense. But your manners could do with some improving. I was only trying to help. I'll ignore your tone this time.

    Ask other people if they have had the patience to go through your messy diagram.
  7. Nov 11, 2007 #6
    I don't want to get in a fight, so I apologize. However, I do feel that you were overly snippy earlier.

    Also, my diagram. is fine. Neatness is not a requisite for proficiency, nor is the diagram important in this case. As I said, I'm sure the formulas are correct, and I wrote them in digitally.

    In any case, this isn't helping anyone. If you can see my problem, please point it out. If not, have a good day and leave me and my drawings alone.
  8. Nov 11, 2007 #7
    No need to fight folks :) why dont we fight the problem :)

    could you make a better draw vidatu? cause i as well have a lil problem reading your draw
  9. Nov 11, 2007 #8

    Hm. I had the arrows going the wrong way when I first uploaded this. All better now.
    Last edited: Nov 11, 2007
  10. Nov 11, 2007 #9

    Shooting Star

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    Hey Vidatu,

    I can assure you that I had absolutely no intention of being snippy. If I made you feel bad, I’m sorry. Let’s forget all this.

    And this diagram is fantastic.
  11. Nov 11, 2007 #10
    No problem. I've had a long day of solving shear/moment diagrams, and should probably go to bed soon. I guess I'm not in the best of moods.

    So, anyway, can anyone see where I messed up?
    Last edited: Nov 11, 2007
  12. Nov 11, 2007 #11

    D H

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    Those solutions don't agree with the relevant equations. Look at the expressions for [itex]\sum F_y[/itex] and [itex]\sum F_z[/itex] with those numbers. Either these numbers or your equations are incorrect.
  13. Nov 11, 2007 #12
    Um, yeah. That's because I messed up typing in the equations. I've fixed that now.
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