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14. waves and sound — Physics STD 11 Science — Question

Gujarat BoardEnglish MediumSTD 11 SciencePhysics14. waves and sound1 Mark
MCQ
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  .... $Hz$
  • ✓
    $720$
  • B
    $555.5$
  • C
    $550$
  • D
    $500$
✓

Answer

Correct option: A.
$720$
a
(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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