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Homework Help: 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
    2. Relevant equations
    doppler effect

    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
    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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    Homework Helper

    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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