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Homework Help: Two Object Kinematics

  1. Jan 28, 2014 #1
    1. The problem statement, all variables and given/known data

    Two balls are thrown upwards from the same spot 1.15 seconds apart. The first ball had an initial velocity of 15.0 m/s and the second was 12.0 m/s. At what time do they collide?

    2. The attempt at a solution

    a = first ball b= second ball

    For them to collide, their height must be the same:

    da = db
    Via x ta + (0.5)(-9.81)(ta^2) = Vib tb + (0.5)(-9.81)(tb)2

    (15 m/s) x (t) + (0.5)(-9.81)(t2) = (12 m/s)(t-1.15) + (0.5)(-9.81)(t-1.15)2

    When I solve for t, I get 0.78 seconds... I know that this is incorrect. I don't know where I went wrong, it might be in how I set up the equation.
  2. jcsd
  3. Jan 28, 2014 #2
    nvm, i've figured it out.
  4. Jan 28, 2014 #3


    User Avatar
    Homework Helper

    setup looks good. I would suspect a mistake in expanding the (t-1.15)² .
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