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Homework Help: Potential Energy and Spring

  1. Apr 18, 2009 #1
    1. The problem statement, all variables and given/known data

    A block of mass m = 3.4 kg is dropped from height h = 44 cm onto a spring of spring constant k = 1920 N/m (Fig. 8-38). Find the maximum distance the spring is compressed.

    [PLAIN]http://edugen.wiley.com/edugen/courses/crs1650/art/qb/qu/c08/fig08_36.gif [Broken]

    2. Relevant equations



    3. The attempt at a solution

    Ok so I found the Potential Energy Ep=mgh. That energy should equal to 1/2kx^2.

    So I set mgh = 1/2kx^2 to solve for x but my answer was wrong?
    1. The problem statement, all variables and given/known data

    2. Relevant equations

    3. The attempt at a solution
    Last edited by a moderator: May 4, 2017
  2. jcsd
  3. Apr 18, 2009 #2
    Your choice to use kinetic and potential energy is correct, but you are missing one small thing.

    The distance the block moves down along the spring (that is, the distance the spring is displaced x) still counts as gravitational potential energy for the block. So modifying the equation you have to take this into account will yield:

    mg(h+x) = .5kx^2

    From here you can solve for x, the displacement of the spring. Since it is a quadratic, you will get 2 different answers, but it should be clear which one is the correct answer.
  4. Apr 18, 2009 #3
    What was your answer for x, I got 0.1236m.
  5. Apr 18, 2009 #4

    that's what I got to the first time, but it was wrong......
  6. Apr 18, 2009 #5
    if you try what I mentioned you will get x=.142m which I believe is the correct answer.
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