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Homework Help: Physics 11 help please?

  1. Dec 18, 2008 #1
    1. The problem statement, all variables and given/known data

    A 1.1 kg cart and a 2.2 kg cart are at rest with a compressed string between them. the force constant of the spring is 600 N/m. when the spring is released, the carts rapidly separate. how far must the spring have been compressed for the 2.2 kg cart to end up moving at 1.5 m/s?
    (involves both momentum and energy)

    2. Relevant equations

    p = mv
    Ek = 0.5mv2
    Ep = 0.5kx2

    3. The attempt at a solution

    i dont even know where to start !!
  2. jcsd
  3. Dec 18, 2008 #2

    Doc Al

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    Staff: Mentor

    Figure out the total KE of the carts. Hint: What's the momentum of the system?
  4. Dec 18, 2008 #3
    ok so i got:
    0.5(1.1)(3^2) = 4.95 J for Cart 1
    0.5(2.2)(1.5^2) = 2.475 J for Cart 2
  5. Dec 18, 2008 #4


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    Gold Member

    Using what you know about the system, apply the law of conservation of energy now.
  6. Dec 18, 2008 #5
    ok so
    = 0.5(7.425)(1.5)2

    but the answer should be 0.16 m.. did i do something wrong or is there another setp now?
  7. Dec 18, 2008 #6


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    Gold Member

    Well, your total kinetic energy is the sum of the kinetic energy of each kart, so KE = 7.425. You know that the resultant KE must equal the initial potential energy so KE = .5(k)(x^2).
  8. Dec 18, 2008 #7
    great thanks !
  9. Dec 18, 2008 #8


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    Gold Member

    You're welcome.
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