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Streching a spring, please help!

  1. Feb 13, 2008 #1
    To stretch a spring a distance 3.09cm from its unstretched length, an amount of work of11.1j must be done. How much work must be done to compress this spring a distance 4.08 from its unstretched length?

    I have been using the equation
    k=Fx/x
    W=(1/2)kx2^2-(1/2)kx1^2

    11.1=(1/2)(Fx/.0309)*(.0309^2)-(0)
    Fx=718N

    So I plug 718N into
    W=(1/2)(718/.0408)*(.0408^2)
    I get 14.6 but it is still not the right answer someone please help
     
  2. jcsd
  3. Feb 13, 2008 #2

    PhanthomJay

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    Science Advisor
    Homework Helper
    Gold Member

    You've put in an unneeded extra step here in solving for F, and in so doing, you have made in error in your last equation. If you solved that k = 718/.0309 in the first step, why are you using k=718/.0408 in the other? The k must be the same.
     
  4. Feb 13, 2008 #3

    Doc Al

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    Staff: Mentor

    You won't need this, since you don't care about the force.
    You'll need this.

    First step: Use the data for the 3.09 cm stretch to find k.
     
  5. Feb 13, 2008 #4
    Thank you, I didnt even notice that
     
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