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Homework Help: Completely Inelastic Collisions

  1. Oct 15, 2007 #1
    1. The problem statement, all variables and given/known data
    A block of mass m1=1.88 kg slides along a frictionless table with a speed of 10.3 m/s. Directly in front of it, and moving in the same direction, is a block m2=4.92 kg moving at 3.27 m/s. A massless spring with a force constant k=1120N/m is attached to the backside of m2. When the blocks collide, what is the maximum compression of the spring? (Hint: at the moment of maximum compression of the spring, the two blocks move as one, find the velocity by noting that the collision is compltely inelastic to this point).

    2. Relevant equations

    3. The attempt at a solution

    I solved for vf in the inelastic equation:

    vf = (m1v1 + m2v2) / (m1 + m2)

    and plugged it into the energy equation and solved for x:

    [(m1v1 + m2v2)^2 / K(m1 + m2)]^1/2 = x

    Plugged in numbers:

    {[1.88(10.3) + 4.92(3.27)]^2 / 1120(1.88 + 4.92)}^1/2 = x

    (1257/7620)^1/2 = x

    x = .407 m = 40.7 cm

    The book got 35.9 cm. Can anyone find my mistake, or if I did it completely wrong, can anyone tell me how to do it? Thanks in advance!
  2. jcsd
  3. Oct 16, 2007 #2
    I did a problem similar to this one from a different text book with the values (everything is worded the same, except these values are in place):

    m1: 2.00 kg
    m2: 5.00 kg
    v1: 10.0 m/s
    v2: 3.00 m/s
    K=1120 N/m

    I approached this problem differenty when I drew the before and after reference frames of what it should look like. And I got:

    vf=5 m/s

    I went on to the energy conservation thing, but tweaked it according to my reference frame:

    m1v1^2 + m2v2^2 + kx0^2 = (m1+m2)vf^2 + kx^2

    Initially, the spring was compressed none, so:

    2(10^2) + 5(3^2) = (7)(5^2) + 1120x^2

    245 = 75 + 1120x^2

    70 = 1120x^2

    x= .25 m = 25 cm. I checked the answer for this problem from the different text book and I got it right. The problem in my original post have values similiar to this problem, but the answer is 35.9 cm. Any chance that they might've messed up? I proceeded to use these very steps on the problem above and wound up with an answer around 24.9 cm, or is it a HUGE coincidence that the way I did it for this problem is wrong, but wound up with the right answer anyways?
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