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

Rajasthan BoardEnglish MediumSTD 11 SciencePhysics14. waves and sound1 Mark
MCQ
A tuning fork vibrating with a sonometer having $20 cm$ wire produces $5$ beats per second. The beat frequency does not change if the length of the wire is changed to $21 cm.$ the frequency of the tuning fork (in Hertz) must be
  • A
    $200$
  • B
    $210$
  • ✓
    $205$
  • D
    $215$
✓

Answer

Correct option: C.
$205$
c
(c) Let the frequency of tunning fork be $N$
As the frequency of vibration string $ \propto \frac{1}{{{\rm{length}}\;{\rm{of}}{\rm{string}}}}$
For sonometer wire of length $20 cm$, frequency must be $(N + 5)$ and that for the sonometer wire of length $21cm,$ the frequency must be $(N -5)$ as in each case the tunning fork produces $5$ beats/sec with sonometer wire
Hence ${n_1}{l_1} = {n_2}{l_2}$

==> $(N + 5) \times 20 = (N - 5) \times 21$

==> $N = 205Hz.$

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