A car moving with a velocity of 36 km/hr crosses a siren of frequency 500 Hz. The apparent frequency of siren after passing it will be

Options

(a) 520 Hz
(b) 485 Hz
(c) 540 Hz
(d) 460 Hz

Correct Answer:

485 Hz

Explanation:

Velocity of the car = 36000 m / 3600 s = 10 m/s
ʋ_emitted = 500 Hz, ʋ_sound = 330 m/s
ʋ_obs = ʋ_emitted [(ʋ_sound – ʋ_obs) / ʋ_sound]
Observed frequency ʋ_obs = 500 [(300 – 10) / 300]
⇒ ʋ_obs = 485 Hz