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Kinematics question on a rock

  1. Jan 14, 2007 #1
    1. The problem statement, all variables and given/known data
    A rock is dropped from a sea cliff, and the sound of it striking the ocean is heard 3.2s later. If the speed of sound is 340 m/s, how high is the cliff.

    2. Relevant equations
    d= v1t + 1/2at^2
    v2^2 = v1^2 + 2ad

    3. The attempt at a solution
    I tried using d = vt since we're given the speed of sound and the time taken for the sound to reach our ears. Since sound is unaffected by gravity, i thought i could straightaway use that formula, but I guess I was wrong. This is how far I got up to. Can someone give me a hand?

  2. jcsd
  3. Jan 14, 2007 #2


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    Staff Emeritus
    Science Advisor

    You can use d=vt for the sound. However, note that we are told that 3.2s after the rock is dropped, we hear it hit the ocean, not that the sound takes 3.2s to reach our ears.

    Use d=vt to obtain the time taken for the sound to reach out ears iafter the rock hits the ocean(in terms of d). We then know that the rock takes (3.2-t)s to fall. Can you write another equation for the rock as it falls?
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