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Homework Help: Vertical Motion w/ constant acceleration

  1. Jan 16, 2010 #1
    1. The problem statement, all variables and given/known data

    A ball is thrown straight up from the edge of the roof of a building. A second ball is dropped from the roof a time of 1.14 s later. You may ignore air resistance.

    A.) If the height of the building is 20.6 m, what must the initial speed be of the first ball if both are to hit the ground at the same time?

    2. Relevant equations

    vf^2 = v0^2 + 2ax
    vf = v0 + at
    x = x0 + v0t + 1/2at^2

    3. The attempt at a solution

    I'm really not sure where to start. If someone could help me out I would greatly appreciate it.
  2. jcsd
  3. Jan 16, 2010 #2


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    Homework Helper

    Focus on the last of your three equations. "t" has to be the same for both objects: the one that's dropped, and the one that's thrown up.
  4. Jan 17, 2010 #3
    Hmm but wouldn't there be two unknowns for that equation? I tried setting the equation equal to itsel but it turned out kind of badly, haha...
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