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Homework Help: Trouble answering this please review!

  1. Oct 18, 2007 #1
    Ive been working this problem for the past hours, and i keep getting the wrong answer... can someone check to see what I am doing wrong, and please correct me..

    An 18g object moving to the right at 26cm/s overtakes and collides elastically with a 33g object moving in the same direction at 16cm/s.

    I need to find the velocity of the slower object and then the faster object.

    My attempt at this question:
    m1v1 + m2v2 = m1v1f + m2v2f
    (.018)(26) + (.033)(16) = (.018)v1f + (.033)v2f
    (.468) + (.528) = (.018)v1f + (.033)v2f
    (.996)= (.018)v1f + (.033)v2f

    then KE cons.
    26-16 = -(v1f-v2f)
    10=v1f + v2f
    v1f = -10 + v2f

    and subsituted (v1f = -10 + v2f) in v1f in the first eq.

    (.996)= (.018)(-10 + v2f) + (.033)v2f
    (.996)= (-.180) + (.018)v2f + (.033)v2f
    (.996)= (-.180) + (.051)v2f
    (1.176)= (.051)v2f
    (1.176/.051) = v2f
    v2f= 23.05882 (((<<< this is for the slower object)))

    For the faster object I work the problem the same way, only using the substitution for v2f this time.

    And Its incorrect. please help!
  2. jcsd
  3. Oct 18, 2007 #2
    You're trying to take a shortcut for the K.E.

    Use : [tex]K.E. = \frac{1}{2}mv^{2}[/tex]

    Then get [tex]v_{1f}[/tex] and [tex]v_{2f}[/tex] using that and you've already got it for the first equation, so plug one of the found velocities into the other equation and it should work out.

    *gulps* I sure hope I'm not messing this up. I should really check before posting; but I'm sure someone will correct me if I'm wrong.
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