1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

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


    User Avatar
    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.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook