Why do the Doppler effect equations yield different frequencies?

[Zeteg]

In the doppler effect equation for sound, measuring frequency, I have the following question:

Car 1-20m/s--> Car 2-30m/s-->
--POINT A--


With the above "diagram", we have car 1 going at 20 m/s, right, and car two, 30m/s right, with respect to point A (doesn't have to be those values, just an example).

If we were to look at car 2, relative to car 1, car 2 would be moving away from car 1 at 10m/s. However, when we plug both the raw values, and the relative values into the equation, they yield a different frequency. Why is this? :|

 

[Andrew Mason]

If we were to look at car 2, relative to car 1, car 2 would be moving away from car 1 at 10m/s. However, when we plug both the raw values, and the relative values into the equation, they yield a different frequency. Why is this? :|

I think you have to take into account the speed of the source relative to the medium as well as the observer. Have a look at this:
http://usna.edu/Users/physics/mungan/Scholarship/DopplerEffect.pdf

AM

 

[Zeteg]

Yes, I'm aware of that. However, why would that be different? In classic motion scenarios, it renders the same answer, does it not? Since sound isn't in any special realms of physics, why is this different?

 

[futb0l]

Well, you would be hearing a different sound while you are in Car 1 and while you are on the ground, would you not?

 

[Andrew Mason]

Yes, I'm aware of that. However, why would that be different? In classic motion scenarios, it renders the same answer, does it not? Since sound isn't in any special realms of physics, why is this different?

The cite I gave you should explain it for you. Think about the case in which the car is traveling at the speed of sound. Does the sound every reach an observer at rest with respect to the medium positioned behind the car? How about in front? So the motion relative to the medium is important.

AM

 

[Zeteg]

The cite I gave you should explain it for you. Think about the case in which the car is traveling at the speed of sound. Does the sound every reach an observer at rest with respect to the medium positioned behind the car? How about in front? So the motion relative to the medium is important.

AM

The site doesn't work =(
I understand your example, I'm still wondering why that is, though. I'll try and figure it out :D

 

[Andrew Mason]

The site doesn't work =(
I understand your example, I'm still wondering why that is, though. I'll try and figure it out :D

It is a pdf file so you need Adobe Acrobat reader installed: http://www.adobe.com/products/acrobat/readstep2.html

AM

 

[Zeteg]

I have that installed, and I can read pdfs :P The link simply does not work for me =\

 

[Andrew Mason]

I have that installed, and I can read pdfs :P The link simply does not work for me =\

You must be copying it and pasting it into your browser. That won't work. You have to click it

AM

 

[Zeteg]

I am clicking it. I know how to operate a computer... ... =x