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 .... $kHz$
Medium
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$f^{\prime}=\left(\frac{v}{v-v_{z}}\right)\left(\frac{v+v_{\mathrm{o}}}{v}\right) f$
$\Rightarrow f^{\prime}=\left(\frac{320}{320-10}\right)\left(\frac{320+10}{320}\right) \times 8$
$\Rightarrow f^{\prime} \approx 8.50 \mathrm{\,kHz}$
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