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Homework Help: Exploding string

  1. Nov 14, 2003 #1
    A massless spring of spring constant 20 N/m is placed between two carts. Cart 1 has a mass M1 = 5 kg and Cart 2 has a mass M2 = 2 kg. The carts are pushed toward one another until the spring is compressed a distance 1.7 m. The carts are then released and the spring pushes them apart. After the carts are free of the spring, what are their speeds?

    a) velocity of car 1 =?
    b) velocity of car 2 =?

    I know that for this problem, I got to use conservation of momentum and conservation of energy, but I don't even know how to even start off this problem. Major help needed!
  2. jcsd
  3. Nov 14, 2003 #2


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    Homework Helper

    You need to apply two equations:
    1. Conservation of energy.
    2. Conservation of momentum.
  4. Nov 14, 2003 #3

    Doc Al

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    Staff: Mentor

    Just give it a try. Here are some hints.

    What's the initial momentum of the system? (Hint: the masses start from rest.) What's the final momentum (m1v1 + m2v2)?

    What's the initial KE of the masses? (see previous hint).

    How much energy is stored in the compressed spring?

    What's the final KE of the masses?
  5. Nov 14, 2003 #4
    Baby steps, right?

    Ok. Initial momentum equals final momentum.
    and I realized objects start from rest, so initial velocity is zero
    initial momentum =0
    so 0=(m1v1+m2v2)
    or -m1v1=m2v2

    Ok, that part wasnt so bad.

    Initial KE=0
    Final KE= .5m1v1^2+.5m2v2^2

    Spring potential energy=.5kx^2

    ah, so yea, I guess then .5m1v1^2+.5m2v2^2=.5kx^2
    Last edited: Nov 14, 2003
  6. Nov 14, 2003 #5

    Doc Al

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    Yep. You're half-way home.
  7. Nov 14, 2003 #6
    Awesome, I figured it out...Lol, it took me like 1 hr of frustration, then like 5 mins of guidance here, and I got it in like 10 mins. :-)
  8. Nov 14, 2003 #7

    Doc Al

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    Staff: Mentor

    Sweet. :smile:
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