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Homework Help: Potential Energy and Force

  1. Jul 18, 2010 #1
    1. The problem statement, all variables and given/known data
    Potential energy function is given by U = C/x, where C is a positive constant.
    a. Find the force as a function of x.
    b. Is this force directed toward the origin or away from it?
    c. Does the potential energy increase or decrease as x increases?
    d. Answer b, and c if C is a negative constant.

    2. Relevant equations
    How do you relate U = C/x to force?

    3. The attempt at a solution
    I tried the formulas.
    F = mg
    U = mgy
    But I am not sure how you relate both of them together
  2. jcsd
  3. Jul 18, 2010 #2


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    welcome to pf!

    hi yamama1! welcome to pf! :wink:

    potential energy is (minus) the work done by a (conservative) force

    in other words, PE = -∫F.dx,

    and so F = … ? :smile:

    (see the pf library for some details)
  4. Jul 18, 2010 #3
    Re: welcome to pf!

    F = ma.
    so PE = - ∫ma.
    I dont know what that would equal to.
  5. Jul 19, 2010 #4


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    (just got up :zzz: …)
    stick to the question … nobody's asked you for a

    you're asked for F, you're given PE, and you have an equation relating PE and F …

    so what is F (as a function of PE)? :smile:

    (oh, and PE isn't -∫ma. it's -∫ma.dx = -∫mv(dv/dx)dx = -∫mvdv = -1/2 mv2)
  6. Jul 19, 2010 #5
    apply F= -dU/dx
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