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Vector Mechanics : Statics

  1. Sep 19, 2015 #1
    1. The problem statement, all variables and given/known data
    92530bd87f.png

    ca36baabd4.png

    2. Relevant equations:
    d = sqrt of dx^2 + dy^2 + dz^2

    3. The attempt at a solution:

    Q1: The second image is the solution diagram. What I don't understand is that why is the force acting from A towards B? Shouldn't it be from B towards A? I'm thinking of it as A as the anchor point which is providing the tension in the cables.

    Q2: In the second image where the length of AB is determined, how are the values for that got? I understand the 80m and the 30m but why is it a -40? The formula that we use is d = sqrt of dx^2 + dy^2 + dz^2. Where the x, y and z are subscripts.

    Thanks.
     
  2. jcsd
  3. Sep 19, 2015 #2
    1) According to Newton's 3rd Law, for every action there is an equal and opposite reaction. Meaning, if you pull or push on something, it will pull or push back. The cable in this problem pulls on point B just as much as it pulls on point A, but they only ask you to consider point A.

    2) If you look at the 2nd diagram, you may see that they consider the positive x, y, and z directions to be in the same direction as the i, j, and k unit vectors. Since you are measuring from A to B, you must measure 40m in the negative x direction to get to B.
    I will admit that the original picture does not make this immediately intuitive, so depending on how you define your axes you may get slight variations in the signage of your answer, but the values should be the same.
     
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