Doppler effect

1. Jun 7, 2003

Dx

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. Jun 7, 2003

dav2008

That should be * not /

3. Jun 8, 2003

HallsofIvy

Staff Emeritus
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.

4. Jun 8, 2003

Dx

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!

<<<(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

5. Jun 8, 2003

dav2008

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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