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Spring energy question

  1. Jun 24, 2005 #1
    Suppose a 300g mass is dropped from a height of 40cm onto a vertical spring with spring constant 200N/m (having a light platform on top) and sticks to the platform (a) How far will the spring compress? (b) How far will the spring be stretched as the mass and spring rebound? use S.I. units.
    I can do part (a) ans = .124m. For part (b) I cannot get right answer =.095m

    Us at the bottom = Ug at end

    where: Us = potential energy of the spring
    Ug = potential energy of gravity

    this gives:

    .5kx^2 = mgx
    => .5kx = mg
    = wrong result
    what am I missing,
    Thanks & regards
  2. jcsd
  3. Jun 24, 2005 #2


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    Science Advisor
    Homework Helper

    If you have done part a) correctly, you know the lowest point of the motion of the mass attached to the spring. If you find the equilibrium point, you can treat the rest of the problem as a harmonic oscillator with an initial displacement starting with zero velocity.
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