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Homework Help: Work Done by Spring

  1. Oct 5, 2008 #1
    1. The problem statement, all variables and given/known data

    An ideal spring is hung vertically from the ceiling. When a 3.0 kg mass hangs at rest from it the spring is extended 9.0 cm from its relaxed length. An upward external force is then applied to the block to move it upward a distance of 12 cm. While the block is being moved by the force, the work done by the spring is:

    -1.2J, -0.52J, -0.26J, 0.52J, 1.2J

    2. Relevant equations

    Hooke's law: F(spring) = -kd
    W(spring) = 1/2kx(initial)^2 - 1/2kx(final)^2
    Wapplied = -Wspring (assuming stationary before and after displacement)

    3. The attempt at a solution

    I think I can plug in 9 as x(initial) and -3 as x(final), giving me:
    W(spring) = 1/2k81 - 1/2k9 = 40.5k-4.5k = 36K
    so W(spring) = 36K

    Not sure what else to do yet
    Last edited: Oct 5, 2008
  2. jcsd
  3. Oct 6, 2008 #2


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    Staff Emeritus
    Science Advisor
    Gold Member

    You need to work out the spring constant and converting from cm to meters might help. Also the spring is not at its natural equilibrium point when the mass is hung. It already has some potential energy.
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