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Homework Help: Having a hard time with visual of FBD

  1. Dec 26, 2007 #1
    I am trying to write a position vector for point D


    I originally thought that the z component was 0...but now common sense is telling me that it can't be....the rope must be sagging.

    Are my first to components correct or am I visualizing this incorrectly? :
    8i+7j-(something)k <---or are my x and y's wrong too?
  2. jcsd
  3. Dec 26, 2007 #2


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    look more carefully at the diagram... (for D) x and y are both smaller than you have stated, and z is not zero.
    It may help to try to redraw the diagram looking from other perspectives, starting with looking downwards.
    Here's a clue... x=3 for point D.
    Last edited: Dec 26, 2007
  4. Dec 26, 2007 #3
    And it is y=2 correct And z=-2?

    I need to look at the fact that the ropes AD and DC are not parellel to the x and y axes so I can't use the 7 and 8 foot measurements that correspond.

    Maybe I should redraw the 3-D case in two 2-D cases instead?
    Last edited: Dec 26, 2007
  5. Dec 27, 2007 #4
    I am so goddamn sick of doing all of these calculations just to get the wrong answer! WTF WTF WTF WTF!
  6. Dec 27, 2007 #5
    Here is my work. This is my third attempt in 3 hours. Am I F'ing stupid? It's okay to tell me. Disregard the attempt to solve the equations. It is a retarded substitution. I am not even sure it would work. But, I now see the easy elimination. But the equations themselves are somehow incorrect.

    When I enter them into an online solver, only T_A comes out correct. I don't know if that helps me figure out where my error is...but this is killing me.

    The equations are the ones marked (1) (2) and (3)

  7. Dec 27, 2007 #6


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    Homework Helper

    Assume the position vector of B as (0,0,0). Wright down the position vectors of A, D, C. and then find vector DA, DB and DC. Find the unit vectors of them. Let p, q and r be the magnitudes of the tensions along DA, DB and DC. consider the points A', B' and C' are the points on the plane containing D which is parallel to the plane ABC. Now take the projection of the three tensions on this plane. For equilibrium two conditions should be satisfied. i) Z- components of the tensions should be equal to 20lb, and projections of the tensions should satisfy the Lami's theorem. You can find out the required angles by noting down the proper distances.
  8. Dec 27, 2007 #7


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    0 5546 Tc is wrong. It should be 0.3244Tc. Try this value.
  9. Dec 27, 2007 #8
    Why? 2/3.606=.5546
  10. Dec 27, 2007 #9
    I have been vindicated! My instructor posted an old solution to the problem in which it was set up differently. My position vectors should be correct.
    Last edited: Dec 27, 2007
  11. Dec 27, 2007 #10


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    rc should be 0i + 7j + 0k and rdc should be -3i + 5j + 2k
  12. Dec 27, 2007 #11
    Thank you sir!

  13. Dec 27, 2007 #12


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    Say, Saladsamurai, what are you using to post your attachments? I'm just getting "red x's" at your illustrations...
  14. Dec 27, 2007 #13
    heh yes I also hate position vectors but there are much worse problems for that then the type you posted. Mid term exam had a equivalent systems question and there were dimensions everywhere, at least 20 dimensions markings - in millimeters as well of course. :cry:
  15. Dec 28, 2007 #14
    Photobucket...is anyone else seeing red x's??

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