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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

Options

  1. A70   Hz
  2. B80 Hz
  3. C75   Hz
  4. D85   Hz

Correct answer

B. 80 Hz

Step-by-step solution

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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