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Homework Help: Potential Energy Function problem (please help)

  1. Sep 29, 2007 #1
    1. The problem statement, all variables and given/known data

    An object moving in the xy-plane is acted on by a conservative force described by the potential-energy function U(x, y)= a*(1 / x^{2}+1 / y^{2}), where a is a positive constant. Derive an expression for the force F expressed in terms of the unit vectors i and j.

    2. Relevant equations

    Gradient vectors, Partial derivatives

    3. The attempt at a solution

    I know I have to take the partial derivatives w.r.t. "x" and "y". But when I did that I came up with F = [(2a)/(x^3)]i + [(2a)/(y^3)]j. But the computer says: "The correct answer does not depend on the variable: a." But if I take the a out, it tells me that: "The correct answer involves the variable alpha, which was not part of your answer."
    Any ideas??
    Thanks in advance
  2. jcsd
  3. Sep 29, 2007 #2


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    Homework Helper

    rename "a" as "alpha" ... maybe "2a" ... ?
  4. Sep 30, 2007 #3
    ^ bump
  5. Sep 30, 2007 #4


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    Homework Helper

    You aren't being ignored: I think we just aren't sure what the computer doesn't like...

    It looks like you've evaluated the gradient of U and expressed the components of F correctly. The issue seems to be what "a" was in the potential function you posted. Is that supposed to be [tex]\alpha[/tex]? Computer entry systems are notoriously finicky. (Curse them!)
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