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Homework Help: 2 body 1 dimension problem

  1. Sep 22, 2004 #1
    Hello, I'm having a bit of trouble starting this problem:
    A ball is dropped from a height of 30 m. At the same instant another ball is thrown straight up from the ground with aspeed of 30 m/s. How long are the balls in the air before they reach the same height?

    I'm not sure what equation to use here. Any help would be appreciated. Thanks in advance.
  2. jcsd
  3. Sep 22, 2004 #2


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    You should have the equations governing the height of each ball.

    The first ball has initial height 30 m and initial speed 0 m/s (it is "dropped"). Its height at any time t (in seconds) is h(t)= 30- (g/2)t2 (g= 9.8 m/s2).

    The second ball has initial height 0 and initial speed 30. Its height at any time t is
    h(t)= 30t- (g/2)t2. "they reach the same height" when h(t)= h(t) for the two formulas: 30- (g/2)t2 = 30t- (g/2)t2. Solve that equation (which is remarkably easy) for t.
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