A police car with a siren of frequency 8 kHz is moving with uniform velocity 36 km/hr towards a tall building which reflects the sound waves. The speed of sound in air is 320 m/s. The frequency of the siren heard by the car driver is

Engineering

Physics

Dopplers Effect

Question

8.25 kHz

7.50 kHz

8.50 kHz

7.75 kHz

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$V_{s} = \frac{36 \times 10^{3}}{3600} m / s = 10 m / s, ν = 8 KHz$

V₀ = 320 m/s

V₀ = V_s = 10 m/s

$ν' = (\frac{V + V_{0}}{V - V_{s}^{'}}) ν$

$= (\frac{320 + 10}{320 - 10}) 8 KHz$

$= \frac{330}{310} \times 8 = 8 .51 KHz$