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Homework Help: Conservation of Energy

  1. Feb 9, 2008 #1
    Mass m=1kg
    Spring constant k=200 N/m
    Unstretched spring length 0.1m

    When the system as show in the attatachment is realeased from rest the spring contracts. Use conservation of energy to determine the magnitude of velocity of the mass when the string and spring are parallel.

    Relevant equations.

    1/2 m v[tex]^{2}_{1}[/tex] + mgy + 1/2 K S[tex]^{2}[/tex] = 1/2 m v[tex]^{2}_{2}[/tex] + mgy + 1/2 K S[tex]^{2}[/tex]

    The attempt at a solution.

    (1/2 * 1 * 0) + (1 * 9.81 * 0) + (1/2 * 200 * ?) = (1/2 * 1 * v[tex]^{2}_{2}[/tex]) + (1 * 9.81 * ?) + (1/2 * 200 * ?)

    I know the initial v=0 and the initial y=0. I'm not sure what I should what numbers to use where the "?" are. These are the final y and the S's. Can anyone help?

    Attached Files:

    • 94.jpg
      File size:
      15 KB
  2. jcsd
  3. Feb 9, 2008 #2
    Still waiting for the picture... but ill elaborate what I can. To begin with, this problem isn't worded well. Rest can be easily mistaken for equilibrium but it is not in this equation. The S of your equation should be the displacement from equilibrium. Although i cant see the pic, hopefully this should be a little more clear.
    Good luck.

  4. Feb 12, 2008 #3
    I hope you can see the pic. I'm not sure what you would consider equilibrium in the pic. I think it would be:[tex]\sqrt{0.45^2 + 0.25^2}[/tex]. Can you advise?
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