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Homework Help: Checking on my answer on wave

  1. May 17, 2006 #1
    I just want to check if I did this question correctly because I'm not really sure if I did.
    The speed of water is 1498 m/s. A sonar signal is sent straight down
    from a ship at a point just below the surface, and 1.80s later the reflected signal is detected. How deep is the ocean beneath the ship?

    First I found the frequency by uing the equation: f= 1/T which is 1/1.80s=.5555555556 Hz

    then I used the equation vel=frequency X wavelength and rearrange it to vel/frequency= wavelength which is (1498m/s)/(.5555555556Hz) and I got 2696.4 m Is this right?
     
  2. jcsd
  3. May 17, 2006 #2

    nrqed

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    To be exact, this is the speed *of sound* in water/
    The 1.80 second is the time for the signal to return to the ship, it has nothing to do with the period of the sound wave itself.

    It's a bit like if I would tell you that a sound pulse traveling at 343 m/s was emitted by a siren, was reflected by a wall at a certain distance and then came back 2 seconds later. It is simply a problem involving speed= distance/time and nothing else.

    You actually do not know the frequency nor the wavelength of the sound wave and you *cannot* calculate it.

    Simply use distance of travel = speed * time and be careful to remember that the distance travelled by the sound wave is twice the depth (because the signal had to do a roundtrip)

    Patrick
     
  4. May 17, 2006 #3
    ohhh i see...hmm the answer turns out to be 2696.4 m as well:smile:

    The time needed for a water wave to change from the equilibrium level to the crest is 0.18s.
    a) what fraction of a wavelength is this?
    I don't get how you get this
    b)What is the period of the wave?
    wouldn't the period of the wave be 0.18s since you do f= 1/T then T= 1/f
    c) What is the frequency of the wave?
    f= 1/T 1/0.18s= 5.5555555556 Hz
     
  5. May 17, 2006 #4

    nrqed

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    Be careful, this is the total distance travelled by the wave, not the depth .
    Draw a sine wave. If you go from the equilibirum position ot the crest, what fraction of a wavelength does that correspond to? I will tell you since I have to go to bed now. It is one fourth of a wavelength. Do you see?
    No, this corresponds to only one fourth of a full wave. So it would take 4 times as long to have a complete oscillation. So the period is 4 times as long
    with the correct period, you will get the correct frequency.

    Patrick
     
  6. May 17, 2006 #5
    i appreciate your help thnx =)
     
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