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Homework Help: Negative sign when finding forces from derivative of potential?

  1. Jun 27, 2014 #1
    Hi! I'm currently reading a book where they give the Coulomb potential, gravitational potential and harmonic potential as




    I think I get the signs as they are used here, but when I am trying to find the force by taking the derivative of these with respect to x, I don't know whether to to f= +dV/dx or f= -dV/dx for any of them and I can't figure out the significance of the sign- which way is defined as +ve/-ve and why?

    Sorry if I posted this is the wrong place or if it is not very clear :/ This is my first post of PF....

    Thank you for any replies! :)
  2. jcsd
  3. Jun 27, 2014 #2
    F = -dV/dx

    This way the force points "down" the slope to a lower potential.
  4. Jun 27, 2014 #3

    Doc Al

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    Staff: Mentor

  5. Jun 27, 2014 #4
    Thank you! :)
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