# GRE Doppler Effect Question

1. Sep 2, 2013

### PsychonautQQ

1. The problem statement, all variables and given/known data
http://grephysics.net/ans/8677/12

I don't understand how it increases it's frequency by a factor of 10, I mean the total velocity goes from 1 to 1.9, therefore It seems like it should increase from 1kHz to 1.9kHz. Why is my 'common sense' wrong?

2. Sep 2, 2013

### Staff: Mentor

What does the Doppler formula give you for the observed frequency when the source is moving? Not sure what you mean by "total velocity"--realize that the source is moving towards the listener.

(I misread the problem myself, the first time through.)

3. Sep 2, 2013

### PsychonautQQ

frequency receiver = (velocity sound + velocity sender) * frequency sender
with velocity of sound = c and velocity of sender = .9c, It seems the frequency received is 1.9 the original frequency. No?

4. Sep 2, 2013

### Staff: Mentor

No.

The Doppler formula for an approaching source is:
$f' = \frac{v}{v - v_s} f$
where $v_s$ is the speed of the source.

See: Doppler Effect

5. Sep 2, 2013

### PsychonautQQ

Okay so the correct Doppler effect the source is in the denominator, I don't understand qualitatively why it matters though. Whether it's the receiver or sender that's movie, the relative velocity between them is going to be the same?

6. Sep 2, 2013

### Staff: Mentor

Note that relative velocity (of source and observer) doesn't enter into the Doppler formula (at least for sound). The speeds are relative to the air. And that a moving source differs from a moving observer. (Compare the two cases, even for the same speed.)

To understand how this is, you'll need to review the details of the derivation of the formula.