An ambulance is racing towards an accident scene, sirens blaring. The ambulance

Question

An ambulance is racing towards an accident scene, sirens blaring. The ambulance is traveling at 90 km/hr and the siren emits a piercing 1000 Hz sound. A police car is already on the scene and the officers are directing traffic. A fire engine is approaching the accident scene from the opposite direction. It is moving at 35 km/hr and has a siren that emits a 500 Hz tone.
a) What frequency does the fire engine emit according to the police officers?
b) What frequency does the ambulance emit according to the ambulance driver?
c) What frequency does the fire engine emit according to the ambulance driver?

Explanation / Answer

Speed of the ambulance is 90 kmph = (90)(5 / 18)m/s = 25 m/s Speed of the fire engine is 35 kmph = (35)(5/18)m/s = 9.72m/s (a) Apparent frequency of the fire engine heard by the police men is f ' = [v / (v - vs)]f = [(340m/s) / (340 m/s - 9.72m/s)](500Hz) = 514.7 Hz (b) Apparent frequency of the ambulance heard by the police men is f ' =[v / (v - vs)]f = [(340 m/s) / (340 m/s - 25 m/s)](1000Hz) = 1079.4 Hz (c) Apparent frequency of the fire engine heard by the ambulance driver is f ' =[(340 m/s + 9.72 m/s) / (340 m/s - 25 m/s)](500Hz) = 555.1 Hz

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