A source and an observer move directly towards each other through still air. The source speed is uₛ, the observer speed is uₒ, the source frequency is f and the speed of sound is v. Which expression gives the frequency heard by the observer?
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Question

A source and an observer move directly towards each other through still air. The source speed is uₛ, the observer speed is uₒ, the source frequency is f and the speed of sound is v. Which expression gives the frequency heard by the observer?

Options

G1 Physics · Accepted Answer

The moving source produces wavelength in front: λ = (v−uₛ)/f. The observer moves towards the incoming wavefronts, so the relative speed of wavefronts past the observer is v+uₒ. Thus, f' = (v+uₒ)/λ = f×(v+uₒ)/(v−uₛ). Correct option: A.