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

Gujarat BoardEnglish MediumSTD 11 SciencePhysics14. waves and sound1 Mark
MCQ
A string $2.0\, m$ long and fixed at its end is driven by a $240\, Hz$ vibrator. The string vibrates in its third harmonic mode. The speed of the wave and its fundamental frequency is
  • A
    $320\, m/s, 120\, Hz$
  • B
    $180\, m/s, 80\, Hz$
  • C
    $180\, m/s, 120\, Hz$
  • ✓
    $320\, m/s, 80\, Hz$
✓

Answer

Correct option: D.
$320\, m/s, 80\, Hz$
d
We have:

$f=\frac{n v}{2 \ell}$

$240=\frac{3 \times v}{2 \times 2}$

$\Rightarrow \quad v=320 \mathrm{m} / \mathrm{s}$

Fundamental frequency $=\frac{v}{2 \ell}=80 \mathrm{Hz}$

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