A tuning fork of frequency $340\,\, Hz$ is vibrated just above a cylindrical tube of length $120 \,\,cm$. Water is slowly poured in the tube. If the speed of sound is $340\,\, ms^{-1}$ then the minimum height of water required for resonance is .... $cm$
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The wavelength of the wave is

$\lambda=\frac{v}{\nu}$

$\lambda=\frac{340}{340}=1 m$

Therefore for different resonances, the required lengths of the air column are

$\frac{\lambda}{4}, \frac{3 \lambda}{4}, \frac{5 \lambda}{4}$ i.e. $25 \mathrm{cm}, 75 \mathrm{cm}, 125 \mathrm{cm}$

As the length of pipe is120cm, the resonance corresponding to $125 \mathrm{cm}$ is not possible.

So, when the water is slowly poured, the first resonance will occur at length

$75 \mathrm{cm}$ of air column. Hence, the height of water is

$h=120-75=45 \mathrm{cm}$

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