A sound source of frequency f moves with speed uₛ. At an instant, its velocity makes an angle θ with the line joining the source to a distant stationary observer, and its component uₛcosθ is directed towards the observer. Which expression gives the instantaneous observed frequency in the usual classical radial-component approximation?
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Question

A sound source of frequency f moves with speed uₛ. At an instant, its velocity makes an angle θ with the line joining the source to a distant stationary observer, and its component uₛcosθ is directed towards the observer. Which expression gives the instantaneous observed frequency in the usual classical radial-component approximation?

Options

G1 Physics · Accepted Answer

For the classical Doppler effect, the relevant source velocity is the component along the instantaneous line joining source and observer. The radial component towards the observer is uᵣ = uₛcosθ. Therefore, f' = f×v/(v−uᵣ) = f×v/[v−uₛcosθ]. Correct option: A.