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The fifth harmonic of a closed organ pipe is found to be in unison with the first harmonic of an open pipe. The ratio of lengths of closed pipe to that of the open pipe is $5/x$. The value of $x$ is _______.

The correct answer is
2

Organ Pipe Harmonics Analysis

This problem involves comparing the frequencies of harmonics in a closed organ pipe and an open organ pipe.

Frequencies of Organ Pipes

  • Closed Organ Pipe: The allowed frequencies (harmonics) are given by the formula $f_n = \frac{nv}{4L_c}$, where $n$ can only be odd integers (1, 3, 5, ...), $v$ is the speed of sound, and $L_c$ is the length of the pipe.
  • Open Organ Pipe: The allowed frequencies are given by the formula $f'_m = \frac{mv}{2L_o}$, where $m$ can be any positive integer (1, 2, 3, ...), $v$ is the speed of sound, and $L_o$ is the length of the pipe.

Harmonic Condition

The problem states that the fifth harmonic of the closed pipe ($n=5$) is in unison (equal frequency) with the first harmonic of the open pipe ($m=1$).

Frequency of the 5th harmonic of the closed pipe: $f_5 = \frac{5v}{4L_c}$.

Frequency of the 1st harmonic of the open pipe: $f'_1 = \frac{1 \times v}{2L_o} = \frac{v}{2L_o}$.

Equating Frequencies

Since the frequencies are equal:

$ \frac{5v}{4L_c} = \frac{v}{2L_o} $

Calculating Length Ratio

We can cancel $v$ from both sides and rearrange the equation to find the ratio $\frac{L_c}{L_o}$:

$ \frac{5}{4L_c} = \frac{1}{2L_o} $

$ \frac{L_c}{L_o} = \frac{5}{4} \times \frac{2}{1} $

$ \frac{L_c}{L_o} = \frac{10}{4} = \frac{5}{2} $

Determining the Value of x

The problem gives the ratio of the lengths as $\frac{L_c}{L_o} = \frac{5}{x}$.

Comparing this with our calculated ratio:

$ \frac{5}{x} = \frac{5}{2} $

Therefore, the value of $x$ is 2.

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