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Static Equilibrium - trick question?

  1. Apr 29, 2015 #1
    1. The problem statement, all variables and given/known data

    A man stands on a board of negligible mass with a length of 10m is supported by 2 cables, one on the left on one on the right, with a tension of 300N and 200N respectively. How much does the man weigh in Newtons?


    2. Relevant equations
    F=ma
    Torque= F*lever arm

    3. The attempt at a solution
    I'm tempted to just 500, but that's far to simple. I thought of setting up a torque equation, but I don't know how I would when I don't know his mass or where he stands. So, if I assume he's standing x meters from the left: 300(x)-200(10-x) + man(mans location) = 0.
    So I set one of the cables as the axis of rotation: the left cable, giving me man(x)=200(10).
    Multiply out to get man(x) = 2000.
    Now I have two variables I can't get rid of.

    But I still really want to just sum all the forces along the y axis and set it equal to zero: 300 + 200 -man = 0, so I still get 500.
    Am I just overthinking the hell out of this?
     
    Last edited: Apr 29, 2015
  2. jcsd
  3. Apr 29, 2015 #2

    haruspex

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    Assuming the cables are vertical, yes, you're overthinking it. It is as easy as it seems.
     
  4. Apr 29, 2015 #3
    No angles were given, so I assume that they are vertical. The question comes after two much more in depth SE problems, hence my confusion. Thanks!
     
  5. Apr 29, 2015 #4

    haruspex

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    Sounds like a classic way to catch people out, follow some tough questions by an easy one with redundant information.
    There's an old geometry problem where you ask someone to cut an L shape, consisting of three squares, into four identical pieces. When they've solved it, you say ok, try this harder one: cut a square into five identical pieces.
     
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