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Homework Help: Two blocks and compressed spring

  1. Oct 23, 2007 #1
    Two blocks with masses m1 = 1.2 kg and m2 = 3.5 kg are at rest on a frictionless surface with a compressed spring between them. The spring is initially compressed by 65.0 cm and has negligible mass. When both blocks are released simultaneously and the spring has dropped to the surface, m1 is found to have a speed of 3.0 m/s.
    What is the speed of m2?
    What is the spring constant of the spring?

    I don't even know how to start this problem. I thought about using the momentum equation, but I'm not sure how to treat the spring.
  2. jcsd
  3. Oct 23, 2007 #2

    Doc Al

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    Staff: Mentor

    If you thought about using conservation of momentum, good! That's what you need. For the second part realize that the final energy of both masses was once stored in the spring.
  4. Oct 23, 2007 #3
    Well I'm trying to figure out how to use conservation of momentum.

    P= 3.0*1.2+5.4*v

    I'm not sure how to find the total velocity becuase I don't have the total momentum.
  5. Oct 23, 2007 #4
    I'm just throwing this out there for you, but since momentum of the system before equals the momentum of the system after...
  6. Oct 23, 2007 #5
    Could I use the potential energy of a spring and set that equal to the kinetic energy of the blocks to figure out the velocity, or do I have to solve the first question in order ot find the spring constant.
  7. Oct 23, 2007 #6
    Honestly, I don't have the whole spring thing down yet, but I think based on the fact that they begin at rest, i.e. momentum before = 0, you can solve for the velocity that you are missing. That get's you the first part. I can't help on the second part as I'd possibly give you bad or misleading advice.
  8. Oct 23, 2007 #7


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    Homework Helper

    Yes, you can.

    Once you find the missing velocity, you can use conservation of energy between the spring and the blocks to find the spring constant.
  9. Oct 24, 2007 #8
    So initial momentum equals zero:


    Does this mean final momentum equals
  10. Oct 24, 2007 #9

    Doc Al

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    Staff: Mentor

    Yes. Realize that the two velocities will have opposite signs because the blocks shoot off in opposite directions.
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