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Question

A tuning fork of frequency 340 Hz is vibrated just above the tube of 120 cm height. Water is poured slowly in the tube. What is the minimum height of water necessary for the resonance?
[Take speed of sound =340 m/s]

A
45 cm
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B
30 cm
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C
25 cm
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D
20 cm
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Solution

The correct option is A 45 cm
We know that v=fλ
λ=vf=340340=1 m
For a closed organ pipe (organ pipe filled with water), it's fundamental resonating length,
l1=λ4=14 m=25 cm
Second resonating length,
l2=3λ4=75 cm
Third resonating length,
l3=5λ4=125 cm
Third resonance is not possible because the length of tube is only 120 cm.
Minimum height of water necessary for resonance,
Hmin=12075=45 cm
Why this question?Tips: The free surface of water will form the closedend of organ pipe. Resonating length is correspondingto standing wave formation.

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