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Homework Help: Question about direction of Moments 3d

  1. Sep 28, 2013 #1
    1. The problem statement, all variables and given/known data

    knowing the tension in cable AB is 570 N, determine the moment about each of the coordinate axes of the force exerted on the plate at B . This link has a picture http://www.chegg.com/homework-help/...oordinate-axes-force-exerted-plate-b-q3031608

    My question is when I am doing the cross product I do r X F . so <900, 0,0> X <450, -350, -180>

    Instead of Fsub AB I do F sub BA so < 900,0,0> X < -450, 350,180> and I get an answer the opposite sign. Does it matter which way I find the force vector? and are both answer equivalent?

    162j-270K or -162j +270K

    I know how to solve the problem I just dont understand why you can use the tension from AB or BA because the answer will have different signs.

    2. Relevant equations

    r X F

    3. The attempt at a solution
  2. jcsd
  3. Sep 28, 2013 #2


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    Yes, it does make a difference.

    The force vector in your first cross product is incorrect, but it is correct in the second cross product.
    The correct force vector for the tension in AB must have a negative x-component.
  4. Sep 28, 2013 #3
    Why does it have to have a negative x component?

    Unit Vector BA =< -450, 300, 180>
    Unit Vector AB=< 450, -300, -180>

    My solution manual is using BA but my teacher is saying it doesn't matter so I am confused. Please explain.. Thanks in advance:)
  5. Sep 29, 2013 #4


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    It matters because you have a tension in line AB. This tension is acting to hold the plate up. If you substituted vector AB, then that force would tend to push the plate in the opposite direction, below the x-axis.

    Are you sure your teacher is saying the direction of the force vector doesn't matter?
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