1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Frequency percieved by observer from emmiter

  1. Oct 25, 2011 #1
    [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?

    Thanks in advance.
     
    Last edited by a moderator: May 5, 2017
  2. jcsd
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Can you offer guidance or do you also need help?
Draft saved Draft deleted



Similar Discussions: Frequency percieved by observer from emmiter
Loading...