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Potential energy equation to force equation?

  1. Oct 17, 2016 #1
    1. The problem statement, all variables and given/known data
    The binding potential energy in a diatomic molecule (N2 for example) is often approximated by the Morse potential energy which has the form U(r) = De (1-e^[-a(r-re)])2 where De is a constant that reflects the well depth (dissociation energy), a is a constant that reflects the width of the well, r is the distance between the atoms, and re is the equilibrium distance. Input an expression for the force as a function of the separation distance (r).

    2. Relevant equations
    F=-du/dx

    3. The attempt at a solution
    F=2a*De*[e^(-2a(r-re))-e^-(a(r-re))]
    I'm just unsure about the re part. I feel like I'm almost correct.
     
  2. jcsd
  3. Oct 17, 2016 #2

    haruspex

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    Looks ok except for the sign.
     
  4. Oct 18, 2016 #3
    Which sign? I entered it into my online homework and it was wrong so that's why I'm asking.
     
  5. Oct 18, 2016 #4
    Seems to be correct.
     
  6. Oct 18, 2016 #5
    Now I have this
    2 a D
    e ( Exp ( - 2 a ( r - re ) ) - Exp ( - a ( r -| re ) ) )
    and I inputted it and it says there's a sign error. I just have no idea where,
     
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