A train approaching a railway crossing at a speed of 120 kmph sounds a whistle of frequency 576 Hz , when it is 288 m away from the crossing. The frequency heard by the observer standing on the road perpendicular to the track from the crossing at a distance of 384 m is (Speed of sound in air = 340 m s - 1 )

Step-by-step solution

The figure below represents the observer, train and crossing. The speed of the observer, u = 120   cos θ u = 120 × 288 384 2 + 288 2 u = 120 × 3 5   km   h - 1 u = 120 × 3 5 × 5 18   m   s - 1 u = 20   m   s - 1 The formula for the frequency heard by observer, when a sound source is moving towards stationary observer, f ' = v v - u   f f ' = 340 340 - 20   576 f ' = 612   Hz