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Homework Help: Velocity of ball problem

  1. Feb 13, 2008 #1
    1. The problem statement, all variables and given/known data
    ball is thrown upward from the ground with an initial speed of 25m/s; at the same instant a ball is dropped from rest from a building 15m high. After how long will the balls be at the same height?

    2. Relevant equations

    3. The attempt at a solution
    well the first ball has initial(vi) of 25m/s and if its thrown that at its peak it wud be 0m/s and the acceleration is -9.81 and i used vf=vi+at which will give me 2.55
    and for the second ball the initial velocity is 0 so i used x=1/2a(t)^2 and i got 1.75

    i have no clue like what to do after you get both the times like how do u find where they would meet up??

    thx for hte help
  2. jcsd
  3. Feb 13, 2008 #2


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    Staff Emeritus
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    Gold Member

    So you have two [kinematic] equations with two unknowns, how would you normally solve a system of simultaneous equations?
  4. Feb 13, 2008 #3
    give an expression for the heigth of the ball as a function of time and then set them equal, and then solve for t.

    in general you have y(t) = y(0) + v(0)t - 1/2gt^2 for an object released at time t=0 at height y(0) with initial speed v(0). you know y(0) and b(0) for both balls.
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