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Static equilibrium/tension force problem!

  1. May 13, 2010 #1
    1. The problem statement, all variables and given/known data

    The attached system is in static equilibrium, and the string in the middle is exactly horizontal.
    Find:
    a)tension T1
    b)tension T2
    c)tension T3
    d)angle theta

    2. Relevant equations

    I know I need to create equations that equal zero to analyze the x and y components of force as well as torque

    3. The attempt at a solution

    So
    Fx=T3x-T1x=0 (I think the tension in T2 is zero)
    Fy=T1y + T3y-m3g-m2g=0
    For the torque equation I'm using the axis of rotation as the point where T1 meets m3

    torque = T2R + T3yR - m2gR

    When I try to solve any of these I come across way too many unknowns to solve...can someone see if I've assessed for forces properly and show me where to go from here?

    Thanks!
    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
     

    Attached Files:

  2. jcsd
  3. May 13, 2010 #2

    K29

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    you have 4 unknowns and 3 equations which means you should be able to solve for each variable fairly easily. ( I say 4 unknowns because T3x cand T3y can both be expressed in terms of T3, with some triangular thinking.. Not sure if your torque equation is right or wrong(haven't done that stuff in a while)
     
  4. May 13, 2010 #3
    Can I assume that T1y=m3g and T3y=m2g?
     
  5. May 13, 2010 #4

    collinsmark

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    Homework Helper
    Gold Member

    Yes, you can and you should. :biggrin: [I assume you mean T1y = (3.0 kg) x g, and T3y = (2.0 kg) x g]

    And it's important to understand why you can. So I encourage you to think about it for a couple of minutes.

    Consider a tiny point on the string, right at the intersection of the T1 string, the horizontal T2 string, and the vertical string holding up the 3.0 kg mass. According to this small point of string, there are only 3 forces acting on it. So you can isolate this particular point to be a function of only these 3 forces. How many forces are acting on that particular point in the y-direction? Since nothing is accelerating, what does it tell you about those forces that exist in the y-direction (as seen from that particular point)?

    [Edit: Btw, The above applies because we are dealing with light strings. If we were dealing with heavy rods, or rigid rods that clamped onto each other and held firm, things might be a little different.]
     
    Last edited: May 13, 2010
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