JEE Main 2014 · 11 Apr · Shift 1 · Physics Q15

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Two factories are sounding their sirens at 800 Hz. A man goes from one factory to other at a speed of 2 m/s. The velocity of sound is 320 m/s. The number of beats heard by the person in one second will be :

Answer and worked solution

Official answer: D

Approaching one siren and receding from the other:
$\Delta f=f\frac{v+u}v-f\frac{v-u}v=\frac{2fu}v=\frac{2\times800\times2}{320}=10\text{ Hz}.$

Topics:Doppler effect Beats

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JEE Main 2014 · 11 Apr · Shift 1, Physics question 15