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Homework Help: Calculating Time, Where Did I Go Wrong?

  1. Feb 14, 2012 #1
    1. The problem statement, all variables and given/known data
    A child accidentally drops her doll from a bridge suspended 50m above a fast moving river. Assuming the doll never reaches terminal velocity, how many seconds will it take for the doll to hit the water?

    2. Relevant equations
    x=x0+ v0t + 0.5*at2

    3. The attempt at a solution
    Using acceleration formula: x=x0+ v0t + 0.5*at2
    It should be 0=(50M)+(0m/s)t+0.5*(-9.8m/s2)
    Which would give me 3.194382825 seconds. The textbook's answer is 2.3s. So after a bit of exploration I realized that the acceleration isn't supposed to be halved. That brings me to the question why isn't the acceleration halved like the example and other questions using the formula?
    Last edited: Feb 14, 2012
  2. jcsd
  3. Feb 14, 2012 #2


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    Gold Member

    Your answer is right (though you made a typo error and forgot a "t" in your equation.)
    The factor 1/2 in front of the acceleration comes from integration.
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