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Homework Help: Freelyfalling bodies again

  1. Jun 9, 2007 #1
    1. The problem statement, all variables and given/known data

    a stone is dropped from rest into a hole. the speed of sound is 343 m/s in air. the sound of the stone striking bottom is heard 1.5s after the stone is dropped, how deep is the hole?



    2. Relevant equations kinematics equations



    3. The attempt at a solution

    not a clue. I know that in 1.5 s it has to drop, hit bottom, and the sound of it travels at 343 m/s back up to vi.:yuck:
     
  2. jcsd
  3. Jun 9, 2007 #2

    Doc Al

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    Staff: Mentor

    You're on the right track, just express it mathematically.
    Total time = time for stone to fall X meters + time for sound to travel X meters.

    (Then you can solve for X.)
     
  4. Jun 9, 2007 #3
    Ok, I understand what your saying, but how do I do this with so many missing variables? if 1.5s = dt + dt...but I dont have either d or t for the stone or sound. or can I somehow use: total T = 1/2 at2(squared) + something with speed of sound...Its just on the tip of my brain! or is my logic way off...
     
  5. Jun 10, 2007 #4

    Doc Al

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    Staff: Mentor

    Hint: The stone and the sound waves both travel the same distance. For the stone: Use falling body kinematics to express the distance as a function of time (t_1). For the sound waves: Express the distance they travel in terms of the speed of sound and time (t_2).

    Now you can put it all together by using these facts:
    (1) The distance traveled is the same
    (2) t_1 + t_2 is given

    Then you can solve for the times and then the distance. Just jump in and try it.
     
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