Two persons $A$ and $B$, each carrying a source of sound of frequency $n$, are standing a few metres apart in a quiet field. $A$ starts moving towards $B$ with a speed $u$. If $v$ is the speed of sound, the number of beats heard per second by $A$ will be
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No. of beats $=\mathrm{n}_{1}-\mathrm{n}_{2}$
${=n\left[\frac{v+u}{v-0}\right]-n} $
${=\frac{n(v+u)}{v}-n=\frac{n u}{v}}$
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