# A railway engine and a car are moving parallel but in opposite direction with velocities 144 km/hr and 72 km/hr respectively. The frequency of engine’s whistle is 500 Hz and the velocity of sound is 340 m s^-1 . Calculate the frequency of sound heard in the car when(i) the car and engine are approaching each other(ii) both are moving away from each other.

1
by LovySankaran569

2015-10-25T15:55:46+05:30

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This change of frequency is called the Doppler effect.

v = velocity of sound

Apparent frequency  f' = f₀  (v +- v_observer) / (v -+ v_source)

1)  f ' = 500 * (340 + 144*5/18) / (340 - 72*5/18) Hz
= 500 * 380/320  Hz

2)  f ' = 500 (340 - 144*5/18) / (340 + 72*5/18)
= 500 * 300/360  Hz