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Homework Help: Doppler Effect of submarines

  1. Jan 29, 2006 #1
    I'm working on the following problem:

    Two submarines are underwater and approaching each other head-on. Sub A has a speed of 4 m/s and sub B has a speed of 18 m/s. Sub A sends out a 1010 Hz sonar wave that travels at a speed of 1522 m/s.

    The question asks two questions, but I'm stuck on the first one:

    What is the frequency of the sound detected by sub B (to the nearest Hz)?

    I was using the formula frequencyobserver = frequencysource * 1 + (vobserver / velocity of sound) / 1 - (vsource / velocity of sound). I assumed that the numerator would use the positive sign since the observer is moving towards the source. I used the minus sign in the denominator since the source is moving towards the observer. For the speed of sound I used 1522 m/s since the submarines are travelling in salt water. I then multiply my calculated numbers times 1010 and got a frequncy of 1019 Hz. However webassign is telling me this is incorrect. Any help with what I'm doing wrong?
  2. jcsd
  3. Jan 29, 2006 #2
    Maybe you pluged in wrong data. Your equation looks correct to me.

    [tex] f' = f \frac{v+v_D}{v-v_S} [/tex]

    [tex] v= 1522 m/s [/tex]

    [tex] v_D = 18 m/s [/tex]

    [tex] v_S = 4m/s [/tex]

    And my answer is f'=1024.6 Hz
  4. Jan 29, 2006 #3
    My goodness, that thing was too picky. I put in 1023 Hz the first time and it didn't accept the number. Thank you for your help.
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