A train moving at a speed of 20 m s - 1 towards a stationary object emits a sound of frequency 930 Hz . Some part of the wave gets reflected from the object, back to the train as an echo. The frequency of this echo, as detected by the driver of the train is [speed of sound in air is 330 m s - 1 ]

Question

A train moving at a speed of 20 m s - 1 towards a stationary object emits a sound of frequency 930 Hz . Some part of the wave gets reflected from the object, back to the train as an echo. The frequency of this echo, as detected by the driver of the train is [speed of sound in air is 330 m s - 1 ], 1400 Hz, 1750 Hz, 1050 Hz, 1550 Hz

MARKS App · Accepted Answer

The speed of the train, v T = 20 m s -1 The speed of sound in air, v = 330 m s -1 The frequency of the echo detected by the driver of the train is v ' = v v + v T v - v T v ' = 930 330 + 20 330 - 20 = 930 × 350 310 Hz ⇒ v ' = 1050 Hz

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