A stationary source (see figure) emits sound waves of frequency f towards a stationary wall. If an observer moving with speed u in a direction perpendicular to the wall measures a frequency f ′ = 11 8 f at the instant shown, then u is related to the speed of sound V s as

Question

A stationary source (see figure) emits sound waves of frequency f towards a stationary wall. If an observer moving with speed u in a direction perpendicular to the wall measures a frequency f ′ = 11 8 f at the instant shown, then u is related to the speed of sound V s as, 3 4 V s, 3 8 V s, 1 4 V s, 8 3 V s

MARKS App · Accepted Answer

This is the case of Doppler's effect in reflected sound. The apparent frequency is given by f ′ = f v + v 0 v - v s Given, f ′ = 11 8 f v 0 = u v s = 0 (stationary source) v = V s ∴ 11 8 f = v s + u cos ⁡ 60 V s 11 8 = 1 + u 2 V s 3 8 = u 2 V s V s = 4 3 u

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