Question:

The fundamental frequency of sonometer wire is \(50\) Hz for some length and tension. If the length is increased by \(25\%\) by keeping the tension same, then the frequency change of second harmonic is

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Frequency is inversely proportional to length.
Updated On: Oct 1, 2026
  • increase by \(20\%\)
  • decrease by \(20\%\)
  • increase by \(40\%\)
  • decrease by \(40\%\)
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The Correct Option is B

Solution and Explanation

Step 1: Understanding the Concept:
For a wire fixed at both ends, the \(n\)th harmonic is \(f_n=\dfrac{n}{2l}\sqrt{\dfrac T\mu}\). At a fixed tension, \(f\propto\dfrac1l\).

Step 2: Second harmonic at the start:
\(f_2=2\times50=100\) Hz.

Step 3: After the change:
The length becomes \(1.25l\), so the frequency becomes \(\dfrac{100}{1.25}=80\) Hz.

Step 4: Percent change:
\[ \frac{80-100}{100}\times100=-20\% \]
The frequency decreases by \(20\%\). Option (B).

Final Answer:
The second harmonic falls from 100 Hz to 80 Hz, a 20 percent decrease. \[ \boxed{\text{Decrease by }20\%} \]
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