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A police car with a siren of frequency 8 kHz is moving with uniform velocity 36 km/hr towards a tall building which reflects the sound waves.The speed of sound in air is 320 m/s.What is the frequency of the siren heard by the car driver?

The general expression for the apparent frequency (n") produced due to Doppler effect is 
n"= n(v+w–vL)/(v+w–vS) where ‘v’ is the velocity of sound, ‘w’ is the wind velocity ‘vL’ is the velocity of the listener, ‘vS’ is the velocity of the source. 
of sound and ‘n’ is the actual frequency of the sound emitted by the source.
here,
n=8khz
v=320m/s
w=0
vL=-36km/hr
   =-36*(5/18) m/s
    =-10m/s
vS=36km/hr
    =10m/s
so the apparent frequency of the siren,n"=8(320+10)/(320-10)
=8*330/310 khz
=8.5khz


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