Question

The siren of a fire engine that is driving northward at 32.0 m/s emits a sound of frequency 2200 Hz . A truck in front of this fire engine is moving northward at 17.0 m/s .

Part A What is the frequency of the siren's sound that the fire engine's driver hears reflected from the back of the truck?

Part B What wavelength would this driver measure for these reflected sound waves?

Answer 1

given

The siren of a fire engine that is driving northward at - 32.0 m/s = V_s

emits a frequency of 2200Hz = F_s  

a truck in front of the fire engine is moving northward at 17.0 m/s = V_L

and velocity V = 344 m/sec

a )

F_t = ( ( V + V_L ) / ( V + V_s ) ) F_s  

F_t = ( ( 344 + 17 ) / ( 344 - 32 ) ) 2200

F_t = 2545.51 Hz

- 17 m/s = V_s , 32.0 m/s = V_L

the reflected frequency is F '

F ' = ( ( V + V_L ) / ( V + V_s ) ) F_s  

F ' = ( ( 344 + 32 ) / ( 344 - 17) ) 2545.51

F ' = 2926.94Hz

b )

= V / F '

= 344 / 2926.94

= 0.1175 m