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  4. A car is moving along X -axis with a velocity v=20 m / s. It sounds a whistle of frequency 660 Hz. If the speed of sound is 340 m / s, the apparent frequency heard by the observer O (shown in figure) is:

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Q. A car is moving along $X$ -axis with a velocity $v=20\, m / s$. It sounds a whistle of frequency $660\, Hz$. If the speed of sound is $340\, m / s$, the apparent frequency heard by the observer $O$ (shown in figure) is:Physics Question Image

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Solution:

$n'=\frac{v}{v-v_{s} \cos 60^{\circ}} n$
Here $v=340\, m / s,\, v_{s}=20\, m / s ,\, n=660\, Hz$
$n'=\frac{340}{340-20 \times \frac{1}{2}} \times 660$
$=\frac{340}{330} \times 660=680\, Hz$