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Physics question

  • 26-05-2009 5:00pm
    #1
    aleyra
    Registered Users, Registered Users 2 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

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


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


  • #3
    sock puppet
    Registered Users, Registered Users 2 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?


  • #4
    aleyra
    Registered Users, Registered Users 2 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


  • #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.


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