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

Gujarat BoardEnglish MediumSTD 11 SciencePhysics14. waves and sound1 Mark
MCQ
A person feels $2.5\%$ difference of frequency of a motor-car horn. If the motor-car is moving to the person and the velocity of sound is $320\, m/sec,$ then the velocity of car will be
  • ✓
    $8 m/s$ (approx.)
  • B
    $800 m/s$
  • C
    $7 m/s$
  • D
    $6 m/s$ (approx.)
✓

Answer

Correct option: A.
$8 m/s$ (approx.)
a
(a) By Doppler’s formula $n' = \frac{{nv}}{{(v - {v_S})}}$
Since, source is moving towards the listener so $n' > n$.
If $n = 100$ then $n' = 102.5$
==> $102.5 = \frac{{100 \times 320}}{{(320 - {v_S})}}$

==> ${v_s} = 8\,m/\sec $

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