# Answer to Question #73475 in Mechanics | Relativity for santhosh

Question #73475

An ambulance siren has frequency 250 Hz. The ambulance is headed towards an

accident site with a speed of 90 km/h. Two police officers on separate motor

cycles head for the same accident site: one follows the ambulance with a speed of

80 km/h and the other approaches the accident site from the other direction with a

speed of 80 km/h. What frequency does ambulance siren has for each of the police

officers? Take the speed of sound equal to 340 m/s.

accident site with a speed of 90 km/h. Two police officers on separate motor

cycles head for the same accident site: one follows the ambulance with a speed of

80 km/h and the other approaches the accident site from the other direction with a

speed of 80 km/h. What frequency does ambulance siren has for each of the police

officers? Take the speed of sound equal to 340 m/s.

Expert's answer

We use the equation for the Doppler Effect:

f=(1+∆v/c) f_0

1) Police officer follows the ambulance

f=(1+∆v/c) f_0=(1+(22.2- 25)/340)250 Hz = 248 Hz

2) Police officer moving towards the ambulance

f'=(1+∆v'/c) f_0=(1+(25 - 22.2)/340)250 Hz = 252 Hz

f=(1+∆v/c) f_0

1) Police officer follows the ambulance

f=(1+∆v/c) f_0=(1+(22.2- 25)/340)250 Hz = 248 Hz

2) Police officer moving towards the ambulance

f'=(1+∆v'/c) f_0=(1+(25 - 22.2)/340)250 Hz = 252 Hz

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