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Homework Help: Two-dimensional oscillators

  1. Sep 22, 2013 #1
    1. The problem statement, all variables and given/known data

    A puck with mass m sits on a horizontal, frictionless table attached to four identical springs (constant k and unstreched length l_0). The initial lengths of the spring a are not equal to the unstretched lengths. Find the potential for small displacements x,y and show that it has the form 1/2 * k' * r[itex]^{2}[/itex] with r[itex]^{2}[/itex] = x[itex]^{2}[/itex]+ y[itex]^{2}[/itex].

    2. Relevant equations

    3. The attempt at a solution

    I'm honestly at a loss with this problem. I know that the total force is F=-kr, where F[itex]_{}x[/itex] = -kx and F[itex]_{}y[/itex] = -ky.

    I also know that my potential is minus the gradient of the force. If I were to take the gradient of F, where F = -kx(i) -ky(j), I get F= -k(i) -k(j). Not really sure where to go from here, or if I'm on the right track for that matter.

    I'm obviously not looking for the answer, just some help in the right direction. I don't think I fully understand it conceptually to be able to work it analytically. Any tips would be greatly appreciated!

    EDIT// Here's some work I've done since posting. Still unsure of how to continue.

    To account for all possible positions of the spring,

    r1^2=x^2 + (a-y)^2
    r2^2=x^2 + (a+y)^2
    r3^2=(x+a)^2 + y^2
    r4^2=(x-a)^2 + y^2

    Now, r will be some variation of the above, so I summed the above 4 equations. I would then think to use U=1/2 * k * r^2, where r^2 is the sum of the above equations. I feel like I'm missing something, though.

    EDIT 2// In fact, I definitely don't feel as though that's suitable, because it says that the potential is not at all dependent on l, which of course is false.
    Last edited: Sep 22, 2013
  2. jcsd
  3. Sep 22, 2013 #2


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    You've got things backwards: the force is the gradient of the potential, ##\mathbf{F} = - \nabla U##. Also, the forces should involve the quantity ##a-l_0##.
  4. Sep 22, 2013 #3
    Whoops, that was a bad mistake. Thanks for catching that.

    Though I'm still not really sure what to do. My potential can then be written in terms of x and y components, right?

    For instance,

    U_x = 1/2 * k * rx^2, but I don't really see what r ought to be. I understand that there should be some dependence on l_0, but I honestly have no idea how. Unless we can essentially rewrite the equations for r_n, with (a-l_0)^2, (a+l_0)^2, etc.

    Sorry, I'm so lost on this problem. I know that there are two methods, one of which involves a taylor expansion, and the other, a second derivative of the potential function, but again, I don't even know what my potential function looks like in this case.
  5. Sep 23, 2013 #4


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    Set up the coordinate system so that the mass is sitting at the origin when it's in equilibrium and the springs lie along the axes. The end of the springs lying on the +x axis will be at the point (a,0). What's the potential energy in the spring when the mass is at (0,0)? What's the potential energy in the spring when the mass is at the point (x,y)?
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