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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 changes by $10 \%$ as a sound source approaches a stationary observer with constant speed $V_s$. What would be percentage change in the frequency as the source recedes the observer with same speed $\left(V_s < V\right)$ is ........... $\%$
  • A
    $10.5$
  • ✓
    $8.5$
  • C
    $4.5$
  • D
    $1.5$
✓

Answer

Correct option: B.
$8.5$
b
(b)

$f^{\prime}=\frac{v}{v-v_s} f_0$

$f$ is such that $f=\frac{110}{100} f_0$

When $\frac{v}{v-v_s}=\frac{110}{100}$

$100 v=110 v-110 v_s$

$v=11 v_s$

When source is received

$\frac{v}{v+v_s} f_0=\frac{x}{100} f_0$

Putting $v=11 v_s$

$\frac{11}{12} \times 100=x$

$x=91.66 \%$

$\%$ change $=100-91.66=8.5 \%$

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