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Question:

On the same path, the source and observer are moving such that the distance between them increases with time. The speeds of the source and observer are the same and equal to 10 m/s with respect to the ground, while no wind is blowing. The apparent frequency received by the observer is 1950 Hz. Then, the original frequency must be (the speed of sound in the present medium is 340 m/s).

602 Hz

2068 Hz

2100 Hz

1903 Hz

Solution:

Since they both move with speed 10 m/s and the distance between them increases with time, they must definitely be moving away from each other. Hence the apparent frequency received by the observer = f₀(c - v₀)/(c + vₛ) = f₀(340 - 10)/(340 + 10) = 1950 Hz ⇒ f₀ = 2068 Hz