A toy-car, blowing its horn is moving with a steady speed of $5 m / s$ , away from a wall. An observer, towards whom the toy car is moving, is able to hear 5 beats per second. If velocity of sound in air is $340 m / s$ , the frequency of the horn of the toy car is :-

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$510 H z$

$680 H z$

$170 H z$

$340 H z$

answer is D.

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Detailed Solution

$n_{1} = [\frac{V}{V - V_{s}}] n = (\frac{340}{340 - 5}) n = \frac{340}{335} n$

$n_{2} = [\frac{V}{V + V_{s}}] n = (\frac{340}{340 + 5}) n = \frac{340}{345} n$

Given : $n_{1} - n_{2} = 5$

$(\frac{340}{335} - \frac{340}{345}) n = 5$

$∴ n = 170 H z$

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