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Homework Help: Height of a Cliff (Only time Given)

  1. Nov 6, 2008 #1
    1. The problem statement, all variables and given/known data
    A person throws a rock off a cliff and hears the splash below 3.4 seconds later. If we know the speed of sound is equal to 340m/s how high is the cliff to the nearest metre?

    2. Relevant equations
    I've been trying to use, r = ut + 1/2at2 and
    v2 = u2 +2ar and
    v= u + at

    3. The attempt at a solution
    I know that the height of the cliff should be around 56 m as:

    r = 0 + 4.9(3.4)2

    But i dont know how to factor in the time delay of the sound :(

    The answer is: (Spoiler)
    52 m

  2. jcsd
  3. Nov 6, 2008 #2


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    Staff Emeritus
    Science Advisor
    Homework Helper

    Welcome to PF!

    Hint: it does not take 3.4 seconds for the rock to fall, it takes slightly less than that.
  4. Nov 6, 2008 #3
    Nice problem, i like it. You should set up a system of 2 equations. One will be the distance when the rock falls down and the other the distance when the sound comes up.
    What is the relation between the two equations and what is the relation between the two times?
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