A source and an observer are situated on two perpendicular tracks as shown in the figure. The observer is at rest and the source is moving with a speed of 50 m s - 1 . The source emits a sound wave of frequency 75 Hz , which travel in the medium with a velocity of 200 m s - 1 . The frequency of the sound heard by the observer when the source crosses the origin is

Question

A source and an observer are situated on two perpendicular tracks as shown in the figure. The observer is at rest and the source is moving with a speed of 50 m s - 1 . The source emits a sound wave of frequency 75 Hz , which travel in the medium with a velocity of 200 m s - 1 . The frequency of the sound heard by the observer when the source crosses the origin is, 70 Hz, 80 Hz, 75 Hz, 85 Hz

MARKS App · Accepted Answer

At the time of emission, if the source and observer line is making an angle θ with the line of motion of source, then f app = f o v v – v s cosθ x = v s t and x = lcosθ [As this sound is heard by observer when source is at O ] ∴ cosθ = v s v and f app = f o v 2 v 2 – v s 2 Putting the values, f app = 80 Hz

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