A train moving at a speed of 220 ms⁻¹ towards a stationary object, emits a sound of frequency 1000 Hz. Some of the sound reaching the object gets reflected back to the train as echo. The frequency of the echo as detected by the driver of the train is: (speed of sound in air is 330 ms⁻¹)

Options

(a) 3500 Hz
(b) 4000 Hz
(c) 5000 Hz
(d) 3000 Hz

Correct Answer:

5000 Hz

Explanation:

Frequency of the echo detected by the driver of the train is (According to Doppler effect in sound)
f’ = (v + u / v – u) f
where, f = original frequency of source of sound
f’ = Apparent frequency of source beacause of othe relative motion between source and observer.
f’ = (330 + 220 / 330 – 220) 1000 = 5000 Hz.