# Frequency percieved by observer from emmiter

1. Oct 25, 2011

### cavpollo

[PLAIN]http://img511.imageshack.us/img511/1282/problemae.jpg [Broken]

This problem is fairly easy, but I just need a little help when it comes to take into account the direction of the moving cars.
(English is not my first language by the way, so bear with me, hehe)

1. The problem statement, all variables and given/known data
There is an ambulance going at 40 m/s to the right and it emits a sound with a frequency of 5kHz. Also, there is a car ahead of the ambulance going at 30 m/s towards the observer.
The speed of sound for calculations should be 343 m/s.
What is the frequency of the sound percieved by the Car?

2. Relevant equations
[PLAIN]http://img689.imageshack.us/img689/564/problema2j.jpg [Broken]

3. The attempt at a solution
Ok, so this is what I did:
1. I substituted the frequency of the emitter by 5kHz and the Speed of sound by 343m/s
2. Then I substituted Vobserver for -30m/s because it is driving away from the source of sound.
3. After that I substituted the Vemitter by 40m/s because it is driving to the right.
4. Cruching the numbers I get that the frequency that the observer gets is of 5.165 kHz

Now here is the Issue I got. My teacher says that instead of using a Vobserver of -30, I should use +30 because of some relative velocity thingy I didn't quite understand (and he didn't have the patience to explain to me... but that is another story).
So he says that my answer should be of 6.155 kHz... but that doesn't make sense to me... I rationalize that to get the value of 6.1kHz the car would be driving towards the ambulance, and not away, as the problem expects.

How wrong am I and why?