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Homework Help: Is the textbook wrong or am I?

  1. Oct 20, 2008 #1
    1. The problem statement, all variables and given/known data

    A vertical spring (ignore its mass), whose spring constant is 875 N/m is attached to a table and is compressed down by .160 m. (a) What upward speed can it give to a .380 kg ball when released?

    2. Relevant equations

    Conservation of Energy using 1/2 k x^2 for Uspring.

    3. The attempt at a solution

    I get 7.68 m/s for the velocity and the book gets 7.47 m/s. Wanted to see who was right and if I'm doing something wrong.
  2. jcsd
  3. Oct 20, 2008 #2


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    "Upward" also means against gravity. You should also figure as an adjustment the m*g*h over the displacement of the acceleration.
  4. Oct 20, 2008 #3
    So you're saying you got the book's answer?

    I used the conservation of energy subbing values for spring and taking into account y=0 when crossing the original spring length. I'm quite sure I have the right answer and the book's is wrong but I just want to verify.
  5. Oct 20, 2008 #4


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    I'm just saying that

    mv2/2 = kx2/2 - m*g*x
    Last edited: Oct 20, 2008
  6. Oct 20, 2008 #5
    You are wrong, your book is right, Pion is right: (except I would not use both "h" and "x", there is only one vertical distance in the problem)
  7. Oct 20, 2008 #6


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    Thanks for the catch. Of course h and x are the same.

    I edited the previous post to be correct now.
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