Advertisement
If you have a new account but are having problems posting or verifying your account, please email us on hello@boards.ie for help. Thanks :)
Hello all! Please ensure that you are posting a new thread or question in the appropriate forum. The Feedback forum is overwhelmed with questions that are having to be moved elsewhere. If you need help to verify your account contact hello@boards.ie

Physics question

Options
  • 26-05-2009 6:00pm
    #1
    aleyra
    Registered Users Posts: 56 ✭✭


    I was just doing some sound revision and i cant figure out how to answer this question. Any help would be great:D

    a car travelling a 10ms-1 approaches and passes a stationary observer. it's horn is continuously sounding. To the observer the frequency changes from 420Hz to 390 Hz.
    Calculate the true frequency and the speed of the sound.

    i think it turns into a quadratic but im not to sure:confused:


Comments

  • Options
    #2
    Dmullar
    Closed Accounts Posts: 67 ✭✭Dmullar


    Use the doppler effect equation F1= fc/(c+-u) as a starting point


  • Options
    #3
    sock puppet
    Registered Users Posts: 4,586 ✭✭✭sock puppet


    I think you do two equations for the object moving away and towards, then express f in terms of c for each and let them equal each other. Do you get a quadratic after that?


  • Options
    #4
    aleyra
    Registered Users Posts: 56 ✭✭aleyra


    yeah that exactly what i did and i end up with a quadratic but im not too sure how i take it from there. i keep getting ridiculously small answers for Fo


  • Options
    #5
    [Deleted User]
    Posts: 4,630 ✭✭✭ [Deleted User]


    f'_toward = cf/(c-v), and f'_away = cf/(c+v)
    420 = cf/(c-10), and 390 = cf/(c+10)
    420(c-10) = cf, and 390(c+10) = cf

    Therefore: 420c - 4200 = 390c + 3900
    Rearrange, and solve for c: 30c = 8100; c = 270

    So, now you've got the speed of the wave.

    Sub it into your original equations:

    420 = 270f/260; 390 = 270f/280

    Solve either one and it'll give you your correct answer: 404.44hz.


Advertisement