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Homework Help: Collision between two moving blocks with a spring

  1. Oct 10, 2012 #1
    1. The problem statement, all variables and given/known data
    A block of mass m1 = 4.0 kg slides along a frictionless table with a velocity of +10 m/s. Directly in front of it, and moving with a velocity of +3.0 m/s, is a block of mass m2 = 9.0 kg. A massless spring with spring constant k = 1120 N/m is attached to the second block as in the figure below. After the collision, the spring is compressed a distance X, find X.

    (Attempt at a drawing)
    block1 Spring.....Block 2
    [m1] |--/\/\/\/[m2]

    Everything is moving in the +x direction, or to the right.

    2. Relevant equations

    3. The attempt at a solution
    I thought that you would use the difference between their KE's to find the PE stored in the spring. However, when I run the numbers into the equation PE = KE1-KE2. I end up getting slightly over. So I'm doing something wrong and I cannot find where it is at.

    Edit: I accidentally hit backspace and reloaded the page, so it freaked out and copied everything.
  2. jcsd
  3. Oct 10, 2012 #2


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    Homework Helper
    Gold Member

    Hello Tadams92,

    Welcome to Physics Forums!
    If you want help, you're going to have to run the numbers and show us your work here. It's part of the forum rules. :smile:
    Again I'm not going to be able to help you until you show your work.

    But just as a general hint, which might possibly relate to what's going wrong, when you are comparing the different kinetic energies (PE = KE1-KE2), you shouldn't be comparing the kinetic energies of Block 1 vs. Block 2 (before the collision). Instead you need to compare the kinetic energy of the entire system before the collision to the kinetic energy of the entire system after the collision at the point in time that the spring is at maximum compression. Conservation of momentum is a necessary part of the process. :wink:
    Last edited: Oct 10, 2012
  4. Oct 11, 2012 #3
    Sorry for the late reply here, but I figured it out today when i asked my physics teacher about it. She basically said the same thing you did collins. compare 2 blocks before and after collision. Thank you for the response though. Good to know I can come here for some help.

    Thanks for letting me know how the forums works a little more. Don't want to be trying to make you guys do my homework for me. I would rather lose a point or two and understand the concept than get the few points. Test points >>> homework points. I'll let you know as much as possible when I ask questions down the road. Thanks again.
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