# Doppler effect

1. Jun 22, 2013

### Asla

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
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?

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2. Jun 22, 2013

### Simon Bridge

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.