Question
Two trains, one coming towards and another going away from an observer both at $4\; m/s$ produce whistle simultaneously of frequency $300 \;Hz$. Find the number of beats produced
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Answer
(c) $\Delta n = \left[ {\frac{v}{{v - u}} - \frac{v}{{v + u}}} \right]\,n = \frac{{2uv}}{{{v^2} - {u^2}}}n$
= $\frac{{2 \times 4 \times 332}}{{{{(332)}^2} - {{(4)}^2}}} \times 300 \approx 7$
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