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Homework Help: Energy/work spring problem.

  1. Dec 6, 2012 #1
    1. The problem statement, all variables and given/known data
    A block of mass 1.20 kg is placed in contact with a spring with a force constant of k = 2.20 N/m. When released, the spring pushes the block and expands a distance of 0.420 meters. If the floor is frictionless, the speed of the block as it leaves the spring is ? m/s.

    2. Relevant equations
    Elastic energy is Ue = .5kd2
    Energy in a block-spring system is E = .5kx2 + .5mv2

    3. The attempt at a solution
    Well the problem is I haven't been able to get anywhere, because in my obviously incorrect thinking, if the block has yet to be released, then all the energy is potential, so E = Ue, but if E = Ue, then .5kx2 = 0, which can't be right. So, in short, I really don't know what I'm doing here. Am I using the wrong equations or something?
  2. jcsd
  3. Dec 6, 2012 #2


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    When the block is not yet released, the spring is compressed by 0.420 m. So its initial PE is not 0.
  4. Dec 6, 2012 #3
    I just solved it, but I think my problem was that I was calculating the initial potential energy while also calculating the final kinetic energy, but still using E = PE, so I ended up with zero on one side and .5mv^2 on the other. I'm not really sure why I did Kf and Ui at the same time, I guess I'm having an off day, haha. But thank you!
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