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Statics cable length problem

  1. Oct 14, 2013 #1
    1. The problem statement, all variables and given/known data

    The window is held open by the cable AB. Determine the length of the cable and express the 30N acting at A along the cable as a Cartesian vector.
    http://img802.imageshack.us/img802/9599/ru5y.jpg [Broken]

    2. Relevant equations

    vector(F) = FuAB

    3. The attempt at a solution

    Here's my solution
    http://img833.imageshack.us/img833/6996/n7b0.png [Broken]
     
    Last edited by a moderator: May 6, 2017
  2. jcsd
  3. Oct 14, 2013 #2

    ehild

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    Check the x and z coordinates of A.

    ehild
     
  4. Oct 14, 2013 #3

    SteamKing

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    Check the coordinates of B.
     
  5. Oct 14, 2013 #4
    Yeah the B coordinates should be B(0,150,250) but i don't know what's wrong with the A coordinates especially the z one.
     
  6. Oct 14, 2013 #5

    ehild

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    The plate is not horizontal, but inclined at 30°angle downward. 300 mm is the length of the edge.

    ehild
     
  7. Oct 14, 2013 #6
    300mm is the length of the edge which is the x coordinate, just like 500 which is the y coordinate.
     
  8. Oct 14, 2013 #7

    gneill

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    Nope. Sketch the side view:

    attachment.php?attachmentid=62900&stc=1&d=13817679824.gif

    EDIT: Added point A to the diagram.
     

    Attached Files:

    Last edited: Oct 14, 2013
  9. Oct 14, 2013 #8
    But isn't the y of point A at the end of the window and not at 0?
     
  10. Oct 14, 2013 #9

    gneill

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    Yes. The sketch is looking along the y-axis (the y-axis is coming straight out of the page), so all y-coordinates will be seen as flat against the x-z plane. Point A is at the end of the heavy line, not at the origin.
     
  11. Oct 14, 2013 #10
    So in this case, what is the coordinate?
     
  12. Oct 14, 2013 #11

    gneill

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    How do you get that? I thought we were talking about the x and z coordinates for point A.
     
  13. Oct 14, 2013 #12

    gneill

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    I modified my diagram to show the location of point A (I thought it was obvious, but it never hurts to clarify). What x and z coordinates would you assign to point A?
     
  14. Oct 14, 2013 #13
    So z=-300sin30 x=300cos30 and y=500.
     
  15. Oct 14, 2013 #14
    No. That was stupid. I edited my post.
     
  16. Oct 14, 2013 #15

    gneill

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    Yes, that looks better.
     
  17. Oct 14, 2013 #16
    Thanks for the help.
     
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