The driver of a car travelling with speed $30$ metres per second towards a hill sounds a horn of frequency $600 Hz$. If the velocity of sound in air is $330$ metres per second, the frequency of the reflected sound as heard by the driver is .... $Hz$
Medium
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(a) From the figure, it is clear that
Frequency of reflected sound heard by the driver.
$n' = n\,\left[ {\frac{{v - ( - {v_o})}}{{v - {v_s}}}} \right] = n\,\left[ {\frac{{v + {v_o}}}{{v - {v_s}}}} \right] = n\,\left[ {\frac{{v + {v_{car}}}}{{v - {v_{car}}}}} \right]$
$ = 600\,\left[ {\frac{{330 + 30}}{{330 - 30}}} \right] = 720\,Hz.$
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