A train moving towards a hill at a speed of $72\ km/hr$ sounds a whistle of frequency $500\ Hz$ . A wind is blowing from the hill at a speed of $36\ km/hr$ . If the speed of sound in air is $340\ m/s$ , the frequency heard by a man on the hill (nearly) is ... $Hz$
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If $\mathrm{n}^{\prime}$ represents the apparent frequency and $\mathrm{n}$ the actual one, then use the relation $\mathrm{n}^{\prime}=\mathrm{n}\left[\frac{(\mathrm{v} \pm \mathrm{w})}{(\mathrm{v} \pm \mathrm{w})-\mathrm{v}_{\mathrm{s}}}\right]$ where $\mathrm{v}$ is the velocity of sound with respect to wind, $w$ is the velocity of wind and $v_{s}$ that of the source. Note that in this case the observer at rest.
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