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Model Paper 7 — Physics STD 11 Science — Question

Rajasthan BoardEnglish MediumSTD 11 SciencePhysicsModel Paper 71 Mark
MCQ
Two sound sources each emitting waves of wavelength $\lambda$ are fixed a given distance apart and an observer moves from one source to another with velocity $u$. Then number of beats heard by him per second is
  • A
    $\frac{2 u}{\lambda}$
  • B
    $\frac{u}{2 \lambda}$
  • C
    $\frac{u}{\lambda}$
  • D
    $\sqrt{u \lambda}$
✓

Answer

(a) $\frac{2 u}{\lambda}$
Explanation:
Image
Number of extra waves received from $S_2$ per second $=+\frac{u}{\lambda}$
Number of lesser waves received from $S _1$ per second $=-\frac{u}{\lambda}$
$\therefore$ Beat frequency $=\frac{u}{\lambda}-\left(-\frac{u}{\lambda}\right)=\frac{2 u}{\lambda}$

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