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Homework Help: Pls help Spring force question

  1. Feb 25, 2009 #1
    The problem statement, all variables and given/known data

    The ends of two identical springs are connected. Their unstretched lengths l are negligibly small and each has spring constant k. After being connected, both springs are stretched an amount L and their free ends are anchored at y=0 and x= (plus minus)L as shown (Intro 1 figure) . The point where the springs are connected to each other is now pulled to the position (x,y). Assume that (x,y) lies in the first quadrant.

    a) What is the potential energy of the two-spring system after the point of connection has been moved to position (x,y)? Keep in mind that the unstretched length of each spring l is much less than L and can be ignored.
    Express the potential in terms of k, x, y, and L.

    b) Find the force F on the junction point, the point where the two springs are attached to each other.
    Express F as a vector in terms of the unit vectors x and y.

    The attempt at a solution

    I can only solve part one.

    For second part,

    my force for the string on the left (lets say force1) = kx1(cos angle1)i + kx1(sin angle1)j and
    my force for the string on the right (lets say force2) = kx2(cos angle2)i + kx2(sin angle2)j

    then i sub in the cos angles and sin angles,

    my total force is 2kLi + 2kyj

    fyi: my i and j are the unit vectors.

    but the answer is wrong. it is independent of L..

    Can help me pls?? Thank you!!

    Attached Files:

  2. jcsd
  3. Feb 25, 2009 #2


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    Homework Helper

    I agree with your force expressions with cos and sin except that the x component in force 1 should be negative. And both y components should be negative.
    F = -2kLi - 2kyj
    Note that the x component has an L in it, so it is not independent of L.
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