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

Gujarat BoardEnglish MediumSTD 11 SciencePhysics14. waves and sound1 Mark
MCQ
A car is moving towards a high cliff. The car driver sounds a horn of frequency $f$. The reflected sound heard by the driver has a frequency $2f$. If $v$ be the velocity of sound then the velocity of the car, in the same velocity units, will be
  • A
    $v/\sqrt 2 $
  • B
    $v/2$
  • ✓
    $v/3$
  • D
    $v/4$
✓

Answer

Correct option: C.
$v/3$
c
Frequency of reflected sound heard by driver $n' = n\,\left( {\frac{{v + {v_O}}}{{v - {v_S}}}} \right)$

It is given that $n' = 2n$

Hence, $2n = n\left( {\frac{{v + {v_{car}}}}{{v - {v_{car}}}}} \right)$ ==> ${v_{car}} = v/3.$

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