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

Gujarat BoardEnglish MediumSTD 11 SciencePhysics14. waves and sound1 Mark
MCQ
The frequency of echo will be $.......Hz$ if the train blowing a whistle of frequency $320\,Hz$ is moving with a velocity of $36\,km / h$ towards a hill from which an echo is heard by the train driver. Velocity of sound in air is $330\,m / s$.
  • A
    $320$
  • ✓
    $340$
  • C
    $360$
  • D
    $380$
✓

Answer

Correct option: B.
$340$
b
The hill will be a secondary source.

$f _{1}=$ frequency of the car w.r.t. the hill

$f_{1}=\left(\frac{v}{v-v_{ s }}\right) f=\left(\frac{330}{320}\right) \times 320=330 Hz$

$f _{2}=$ Frequency of the sound reflected by hill w.r.t. the car $(echo)$

$f_{2}=\left(\frac{v+v_{0}}{v}\right) f_{1}=\frac{(330+10)}{330} \times 330=340\,Hz$

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