Doppler Effect and a Trumpet

A person on a railroad car blows a trumpet sounding at 485.0 Hz. The car is moving toward a wall at 21.0 m/s. assume the speed of sound is 343.0 m/s
Calculate the frequency of the sound as received at the wall.
Calculate the frequency of the reflected sound arriving back at the source.

case 1 :

Using DOPLER effect equation F'=[( V-Vo) / (V-Vs)] (f)
HERE Vo=velocity of observer , Vs=velocity of source, V=343 m/s and f=FREQ of trumpet.

here Vo=0, Vs=21 m/s
so F'={ (343-0) / (343-21) ](485)
= 516.6 Hz

Case 2:

HERE weve to consider the source as wall and the car as observe as reflected sound is heard by car.
source freq=reflected wall freq= 516.6 (from case1)
so F''=[ (V-Vo) / (V-Vs) ](516.6)
substituting F''=[ (343-(-21)) / (343-0)] (516.6)
=548.22 HZ

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