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How To Find Depth Given TIME ONLY

  1. Oct 18, 2011 #1
    1. The problem statement, all variables and given/known data
    The time it takes a high frequency sound to travel from the surface of the lake to the bottom of the lake and back to the surface is 0.1seconds. What is the depth of the lake?


    2. Relevant equations
    Speed of sound in water= 1500m/s
    Time=0.1s
    Depth=?


    3. The attempt at a solution
    Depth= Speed of sound * time elapsed/2= 1500*0.1/2
    = 75m
    IS THIS CORRECT? PLEASE HELP?
     
  2. jcsd
  3. Oct 18, 2011 #2

    phyzguy

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    Looks good to me!
     
  4. Oct 18, 2011 #3
    Really? I assumed that 1500 m/s was the speed of sound in water. Didn't know if it was correct since it wasn't given to me in the question... Can u help me with another question please?
     
  5. Oct 18, 2011 #4
    Really? I assumed that 1500 m/s was the speed of sound in water. Didn't know if it was correct since it wasn't given to me in the question... Can u help me with another question please?
     
  6. Oct 18, 2011 #5

    phyzguy

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    Oh, I assumed you were given the speed of sound in water at 1500 m/s, but it still looks good, since Wikipedia says the speed of sound in water is 1484 m/s (http://en.wikipedia.org/wiki/Speed_of_sound). What is your other question?
     
  7. Oct 18, 2011 #6
    Thanks.
    The other question is:
    A ship is 220metres from a large cliff when it sounds its foghorn. If the echo time is 0.5s, how far is the ship from the cliff?

    I don't understand what to do.
     
  8. Oct 18, 2011 #7

    phyzguy

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    It's conceptually the same as the first one. Treat it the same way. What is an echo?
     
  9. Oct 18, 2011 #8
    An echo is a repeated sound caused by the reflection of sound waves from a surface. I still don't get what to do :(

    Should I multiply 220 by the 0.5? :/
     
    Last edited: Oct 18, 2011
  10. Oct 18, 2011 #9

    phyzguy

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    An echo means the sound went from the ship to the cliff, was reflected from the cliff and traveled back to the ship, where it was heard. Does this help?
     
  11. Oct 18, 2011 #10
    I understand that but I just don't know how to work it out.. would the distance be the same?
     
  12. Oct 18, 2011 #11

    phyzguy

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    How far does the sound have to go to go from the ship to the cliff and back?
     
  13. Oct 18, 2011 #12
    220metres...
     
  14. Oct 18, 2011 #13

    phyzguy

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    If the ship is 220 meters from the cliff, how far does the sound have to go to go from the ship to the cliff and back?
     
  15. Oct 18, 2011 #14
    440 metres?
     
  16. Oct 18, 2011 #15

    phyzguy

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    OK, good. Now how fast is the sound traveling (remember it is in air)? And given this, how long will it take?
     
  17. Oct 18, 2011 #16

    DaveC426913

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    1] this was posted elsewhere and,
    2] taken as-is, it's a trick question. The answer is provided in the question.
     
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