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Doppler effect

  1. Jun 22, 2013 #1
    1. The problem statement, all variables and given/known data
    An ambulance is moving in an expressway at a speed of 60km/hr from East to West(from A to B) as shown in the figure with a siren of 880Hz. Let the frequecies of the siren sound detected by an observer located at point O be Va and Vb as the ambulance just passes point A and B respectively.Point C is just to the North of O and OC=AC=BC=100m. Here the sound velocity in calm air is 340m/s.
    i)Incase there is an east wind of 5m/s(18km/hr), what is the approximate value of (va/vb)-1
    i)Incase there is a North wind of 5m/s(18km/hr),what is the approxiamate value of (va/vb)-1
    attachment.php?attachmentid=59776&d=1371910117.png
    2. Relevant equations
    doppler effect
    f'=(v+vs)/(v°-v)f

    3. The attempt at a solution
    I ignored the frequency values and concentrated on the ratios of change of frequency but I cannot comprehend how the frequency will change in the vb case.As for Va the wind will definitely reduce the frequency so that the ratio will be
    (340+50/3+5)/340.
    could someone help me on how the frequency for vb will change?
    Do the distances given have any effect on the question?
     

    Attached Files:

  2. jcsd
  3. Jun 22, 2013 #2

    Simon Bridge

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    Hint:
    When the wind is coming from the east, so the sound from B has to travel against the wind to get to O.
    It can help to sketch in the whole wave-fronts.
     
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