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Upward velocity of a block on a string

  1. Sep 29, 2011 #1
    1. The problem statement, all variables and given/known data
    The force constant of a spring is 600 N/m and the un-stretched length is 0.72m. A 3.2kg block is suspended from the spring. An external force slowly pulls the block down, until the spring has been stretched to a length of 0.86m. The external force is then removed, and the block rises. In this situation, when the spring has the contracted to a length of 0.72m, What is the upward velocity of the block?

    2. Relevant equations
    conservation of energy

    3. The attempt at a solution

    KEinitial + Uinitial = KEfinal + Ufinal
    0 + 0.5(600)(0.14)^2 = 0.5 x 3.2 x v^2 + 0
    v = 1.92m/s?
     
  2. jcsd
  3. Sep 29, 2011 #2

    gneill

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    Staff: Mentor

    Don't forget that gravity is working on the block too!
     
  4. Sep 29, 2011 #3
    KEinitial + Uinitial = KEfinal + Ufinal
    0 + 0.5(600)(0.14)^2 + (3.2)(9.8)(0.14) = 0.5 x 3.2 x v^2 + 0

    Is this right?
     
  5. Sep 29, 2011 #4

    gneill

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    Staff: Mentor

    Better, but should the gravitational PE be negative? (It looks like you're taking the zero reference for it at the unstretched spring level).
     
  6. Sep 29, 2011 #5
    Thank you, that is what i am missing out.
     
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