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

Gujarat BoardEnglish MediumSTD 11 SciencePhysics14. waves and sound1 Mark
MCQ
A small source of sound moves on a circle as shown in the figure and an observer is standing on $O$. Let ${n_1},\;{n_2}$ and ${n_3}$be the frequencies heard when the source is at $A,\,B$ and $C$ respectively. Then
  • A
    ${n_1} > {n_2} > {n_3}$
  • ✓
    ${n_2} > {n_3} > {n_1}$
  • C
    ${n_1} = {n_2} > {n_3}$
  • D
    ${n_2} > {n_1} > {n_3}$
✓

Answer

Correct option: B.
${n_2} > {n_3} > {n_1}$
b
(b) At point $A$, source is moving away from observer so apparent frequency ${n_1} < n$ (actual frequency) 

At point $B$ source is coming towards observer so apparent frequency ${n_2} > n$

and point $C$ source is moving perpendicular to observer so ${n_3} = n$ 

Hence ${n_2} > {n_3} > {n_1}$

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