Question 60: Waves

Question

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. The frequency of the siren
heard by the car driver is

Accepted Answer

u = 36 km h
$$-$$1
= 10 ms
$$-$$1
,
$$v$$ = 320 ms
$$-$$1
, $$
u $$ = 8 kHz
The sound reflected from the building may be imagined to be coming from the mirror image. The driver is approaching the image-source which is also approaching him with the same speed. Hence the frequency of sound heard by the driver is
$$
u $$' = $$
u $$ $$\left( {{{v + u} \over {v - u}}} ight)$$
= 8 kHz $$	imes$$ $$\left( {{{320 + 10} \over {320 - 10}}} ight)$$ = 8.5 kHz

Answer

8.50 kHz

Answer

8.25 kHz

Answer

7.75 kHz

Answer

7.50 kHz

Waves

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. The frequency of the siren heard by the car driver is

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