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Doppler effect (source and detector moving together)

  1. Dec 17, 2016 #1
    1. The problem statement, all variables and given/known data

    A source emits sound with a frequency of 860 Hz. It is moving at 20.0 m/s toward a stationary reflecting wall. If the speed of sound is 343 m/s, what frequency does an observer riding with the source hear?


    • A

      860 Hz



    • B

      913 Hz



    • C

      910 Hz



    • D

      765 Hz


    • E

      967 Hz

    2. Relevant equations
    CodeCogsEqn-4.gif
    WHERE FL and vL are the frequency and velocity of sound heard by the personL
    fs and vs are the frequency and velocity of sound emmited by the source

    3. The attempt at a solution:
    v=343 m/sec
    vs=20 m/sec
    vL=-vs=-20 m/sec since if it is moving away from the source
    fs=860Hz
    then using the aboove equation
    CodeCogsEqn-4.gif
    FL=860Hz=Fs
    but it is incorrect? Why?
     
  2. jcsd
  3. Dec 17, 2016 #2

    Doc Al

    User Avatar

    Staff: Mentor

    You need to do this in two steps. Step one: What is the frequency received at the wall? (Then treat the reflected sound from the wall as the new source.)
     
  4. Dec 17, 2016 #3
    Ah then vs=vL since both are coming toward each other.
    then fL=976 Hz which is correct :) thanks
     
  5. Dec 17, 2016 #4
    Suppose we don't have a wall, then how are we supposed to find that frequency received by the person?
     
  6. Dec 17, 2016 #5

    Doc Al

    User Avatar

    Staff: Mentor

    Not sure what you mean. What is he hearing? (There needs to be a source of sound.)
     
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