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Homework Help: Doppler effect

  1. Jun 7, 2003 #1

    Dx

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    Using the doppler effect: formula: f'=f/(1-V_s/V_0)

    A train traveling toward you at 120k/h. the train blows its 400Hz whistle. take the speed of sound to be 340m/s. whats the freq?

    my developments so far...
    400Hz/(1-(.340/.343ms)

    how do i solve the setup is wrong?
     
  2. jcsd
  3. Jun 7, 2003 #2

    dav2008

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    That should be * not /
     
  4. Jun 8, 2003 #3

    HallsofIvy

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    I have no idea what "how do i solve the setup is wrong?" means- unless you are combining two sentences. Are you related to James Joyce?

    However, you would complete the problem by just doing the arithmetic:
    From the formula f'=f/(1-V_s/V_0)
    you get f'= 440HZ/(1- 210/340) assuming that V_0 is the speed of sound and that V_s is the speed of the source of the sound.
    (I might have been inclined to take V_s as the speed of Sound but if V_s> V_0 you get a negative number for f'!)

    (you give "400Hz/(1-(.340/.343ms)" I don't know why you have the decimal points nor do I know where you got "343".)

    Notice that since V_s and V_0 both have units of speed (m/sec), the units in the fraction V_s/V_0 cancel so that both it and the denominator 1- V_s/V_0 are "dimensionless". f' will have units of
    Hz just like f.

    I get that V_s/V_0= 120/340= 0.353 (I am taking three significant figures since the speeds are given to three significant figures.),
    1- V_s/V_0= 1- 0.353= 0.647 and f'= 440/0.647= 525 Hz.
     
  5. Jun 8, 2003 #4

    Dx

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    Try Again Ivy!

    <<<I have no idea what "How do i solve? The setup is wrong?" means- unless you are combining two sentences. Are you related to James Joyce? Since we want to be corect here Ivy! :wink:

    <<<(you give "400Hz/(1-(.340/.343ms)" I don't know why you have the decimal points nor do I know where you got "343".)

    Let me enlighten you that .343 is equal to 343x10^-3m/s which is the spd of sound in air at 20 degrees C. I want you to also be corect since you could be related to Mr Amison my Biology teacher who know absolutely nada about science too.

    <<<I get that V_s/V_0= 120/340= 0.353 (I am taking three significant figures since the speeds are given to three significant figures.),
    1- V_s/V_0= 1- 0.353= 0.647 and f'= 440/0.647= 525 Hz.

    Lastly, H.O.I., sir. I want to say i appreciate your response and help thus far but your answer is 100% incorrect. Thank You! Please don't respond to my threads unless your serious about helping me. Your intentions are sensere and noble but i wanna learn this stuff, ok!

    Thanks everyone! BTW...I double checked my formula is correct in the book.
    Dx :wink:
     
  6. Jun 8, 2003 #5

    dav2008

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    yeah, my bad..i just saw that division sign and thought that was the problem, but i guess i just learned the formula as multiplying by the reciprocal..instead of your form..
     
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