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Homework Help: Massless beam held against a wall

  1. Dec 14, 2012 #1
    1. The problem statement, all variables and given/known data
    Just took my finals and there was one problem that bugged me, I have done these types of problems multiple times and understand it well but this particular problem seems like there isn't enough information.

    There is a massless beam being held against a wall by a massless string. A 2000N mass is attached to the end of the beam. The beam is L meters in lenght. The distance from the string on the wall to the end of the beam touching the wall is L. The beam is θ degrees above the horizontal.
    a) Find the tension in the string
    b) Find the angle θ
    c) Find the horizontal and vertical force being exerted on the massless beam by the wall.

    Basically the only value you have is 2000N, and you know that the distance of the beam and the distance of the wall is equal, and therefore it forms an isosceles triangle.

    I couldnt find a picture of the problem but the set up is similar to this
    except the bar is tilted at angle θ, the lenght of the bar is L.

    2. Relevant equations

    3. The attempt at a solution

    I understand the equations and isolation of systems completely, the only thing that is restricting me from solving the problem is finding the value of θ so I was wondering if anyone who is really good at trig could help.
  2. jcsd
  3. Dec 14, 2012 #2


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    There is clearly not enough information here. Are you sure you have this verbatim?
  4. Dec 14, 2012 #3
    I thought so, yes this is verbatim. All of my peers in class wern't able to solve it either so I'm sure there isn't enough information. Just wanted to see if someone with higher knowledge thought the same.
  5. Dec 14, 2012 #4
    It doesn't seem completely unsolveable. I've done similar problems. Let's see what equations we can set up... forces in y direction (equal to 0), forces in x direction (also 0), balancing of torques (also 0) and maybe some trig equations: addition of angles and the law of cosines *could* possibly help.
  6. Dec 14, 2012 #5


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    One thing that intrigues me is this phrase: "being held against a wall by a massless string". What exactly does that mean? Only one end of the beam touches the wall. It almost suggests it's just resting against the wall rather than hinged, but there would have to be some friction or the beam would slip (unless theta=0), so it might as well be hinged. Maybe it is supposed to be frictionless and hingeless, in which case it is easy to solve.
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