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Homework Help: Potential Energy

  1. Jan 18, 2010 #1
    1. The problem statement, all variables and given/known data
    A spring is compressed a distance of 1m and a 2 kg rock is shot straight up with the spring. The rock attains a maximum height of 500m. What is the force constant k of the spring?

    Use g=10m/s^2

    2. Relevant equations
    P.E.= mgh
    P.E. elastic= (1/2)(k)(x^2)

    3. The attempt at a solution

    P.E.= mgh
    P.E.=(2000g)(10m/s^2)(500m)
    P.E.= 10000000 newtons

    P.E. elastic= (1/2)(k)(x^2)
    k= P.E elastic/ (1/2)(x^2)
    k= 10000000/ (1/2)(1)
    k=2000000 newtons ?
     
  2. jcsd
  3. Jan 18, 2010 #2
    Use SI units ie kilogram, not gram. Furthermore, energy is in joules, not newtons! Also, the spring constant k is in newtons per meter, not newtons. Please read up on units, okay?
     
  4. Jan 18, 2010 #3
    Thanks,

    So, if I correct my units I would have:

    P.E.= mgh
    P.E.=(2kg)(10m/s^2)(500m)
    P.E.= 10000 joules

    P.E. elastic= (1/2)(k)(x^2)
    k= P.E elastic/ (1/2)(x^2)
    k= 10000/ (1/2)(1)
    k=20000 N/m ?

    Is this correct?
     
  5. Jan 18, 2010 #4
    Looks good; that is one very strong spring indeed.
     
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