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Homework Help: Work done by spring

  1. Mar 22, 2009 #1
    1. The problem statement, all variables and given/known data
    A spring has a relaxed length of 7 cm and a stiffness of 50 N/m. How much work must you do to change its length from 12 cm to 16 cm?

    2. Relevant equations

    3. The attempt at a solution
    1/2*50*(.16-.12)2 = 0.04

    It shows the solution to be 0.14. It says the change in the energy of the spring is its change in potential energy, so the change in (1/2)kss2. Be sure to use the stretch s, not the length, and use meters, not centimeters.
  2. jcsd
  3. Mar 22, 2009 #2


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    Homework Helper

    Looks like an algebra error and a misunderstanding of stretch.
    Length 12 is a stretch of 12 - 7 = 5. Length 16 is a stretch of 16 - 7 = 9.
    .5k(.09)^2 - .5k(.05)^2 is not equal to .5k(.09-.05)^2.
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