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Need help solving 3D

  1. Sep 21, 2015 #1
    It statics, I am having difficulty on getting perfect to read the dimensions correctly in order to find the forces!

    This photo for example

    media%2F942%2F942137b8-2360-4fe4-8258-a6d7d240e8ea%2Fimage.jpg

    I put B = (-3i,-2j, -6k) but correct answer is (3i,-2j,-6k) ... why 3i but not 3i ?! I'm reeally confused

    For C I put -1i but correct answer is 3i !.. how?!???

    for D I added the 1 and 2 and i put -3i , and I put -3j but correct is 3j ....

    can anyone explain and give me easier way to approach the answer and avoid mistakes?! I have an exam on friday and this is basic thing that i cant seem to understand!
     
  2. jcsd
  3. Sep 21, 2015 #2

    RUber

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    For B, your positive x is coming toward you. Use the right-hand rule to keep your positive directions straight.
    C looks like it is at -3i -1j .
    D looks like it is at -2i and 3j.
    Your i components need to find references on the x axis. Your j components need references on the y axis.
     
  4. Sep 21, 2015 #3
    I dont get it
     
  5. Sep 21, 2015 #4

    RUber

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    Look at C. The left-right line above it says that it is 1ft left of the x-axis. That means the y-component is -1.
    The long line going through C crosses the segments marked 1ft and 2ft by D. This means that C is 3ft behind the y-axis, or the x component is -3.
    Does that help?
     
  6. Sep 24, 2015 #5
    I understand now after doing 99999 researches and watching 9999 videos... and easier and simple is to take AB for example, find points A and B, then subtract A-B and BINGO!
     
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