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Homework Help: Work Done By Spring

  1. Jun 10, 2008 #1
    1. The problem statement, all variables and given/known data

    The question is:
    A spring is hung from a roof. Uncompressed it is 40cm long. The spring is known to have a spring constant of 10Nm-1
    A ball of mass 0.2kg is hung from the spring which extends to 60cm. (therefore extension is equal to 20cm)
    (Note the ball is stationary not oscillating)
    Calculate energy stored in the spring.

    2. Relevant equations
    Now to solve this is took the change in gravitational potential energy to find the energy absorbed by the spring. Using E=mgh

    A friend used the energy of a spring equation which is E=0.5kx^2

    3. The attempt at a solution

    Using the change in potential energy I produced the answer of 0.4 J
    Using the spring equation an answer of 0.2 J

    Which is correct? I'm sure one is incorrect because it does not account for gravity or something along those lines but can't think of a way to show it.
  2. jcsd
  3. Jun 11, 2008 #2


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    Homework Helper

    And this is correctly calculated
    ( approx. 0.2 kg · 10 m/(sec^2) · 0.2 m = 0.4 J ). However, it is not the answer to the question. While this is the change in the gravitational potential energy of the ball-Earth system, it does not measure the potential energy stored in the spring.

    The change in the spring potential energy
    ( [1/2] · 10 N/m · [0.2 m]^2 = 0.2 J ) is what the question appears to ask for, so this is probably what should be given for the answer.

    The apparent discrepancy seems to be due to the fact that each of you is looking at something different in the overall system. Consider that the ball is subject to two forces: the downward force of gravity and the upward force from the spring. So the work on the ball done by gravity over the downward path is +0.4 J ; the work done by the spring over the same path is -0.2 J . The work done by gravity will be equal to the opposite of the change in gravitational potential energy, which is m · g · delta_y = -0.4 J . There is no net change in the kinetic energy of the ball, so the 0.4 J made available from the gravitational field is taken up in part (0.2 J) by the work done by the spring to keep the ball from accelerating and the rest (0.2 J) goes into the spring's potential energy (at the microscopic level, it goes into stretching the bonds between the atoms in the spring's structure, which is often loosely referred to as "tension").
    Last edited: Jun 11, 2008
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