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Homework Help: Conservation of Energy- spring

  1. Nov 17, 2008 #1
    1. The problem statement, all variables and given/known data
    A 6kg block slides on a horizontal frictionless surface with a speed of 1.2 m/s. It is brought momentarily to rest when it hits a bumper that compresses a spring.

    Heres a picture of the problem.

    When I was doing this problem, I thought there were only two different energies: the Kinetic Energy and the Elastic Potential Energy. I assumed there was no Gravitational PE b/c there was no height involved. Therefore, I assumed (according to Law of Conservation of Energy) that you could set the Kinetic Energy equal to EPE.
    (1/2)mv^2 + (1/2)kx^2= (1/2)mv2^2 + (1/2)kx2^2 which reduces to
    (1/2)mv^2= (1/2) k(-Y2)
    I am using -Y to denote the compression of the spring
    if I plug in and solve
    (1/2)(6kg)(1.2m/s ^2)= (1/2) (4)(Y2)
    I get 1.341640786 m as the amount of compression. However, this answer is wrong. Can someone tell me what I did wrong?
  2. jcsd
  3. Nov 17, 2008 #2


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    Homework Helper

    Check your units. Spring constant is 4K*N/m
  4. Nov 17, 2008 #3
    Oh wow that was stupid of me. Thanks!
  5. Nov 17, 2008 #4
    Your understanding of the physics is correct, but perhaps you should check if your spring potential energy term with Y_2 is correct. Energy has units of Newton x meter, and the spring constant has units of Newton per meter.
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