An organ pipe $40 \mathrm{~cm}$ long is open at both ends. The speed of sound in air is $360 \mathrm{~ms}^{-1}$. The frequency of the second harmonic is ___________ $\mathrm{Hz}$.

<p>An organ pipe that is open at both ends resonates at all harmonics, including the fundamental (first harmonic), second harmonic, third harmonic, etc.</p> <p>The frequency $f$ of the $n$-th harmonic for a pipe open at both ends is given by:</p> <p>$f_n = \frac{n v}{2L}$,</p> <p>where:</p> <ul> <li>$n$ is the number of the harmonic,</li> <li>$v$ is the speed of sound, and</li> <li>$L$ is the length of the pipe.</li> </ul> <p>To find the frequency of the second harmonic ($n = 2$), we can substitute the given values into the formula:</p> <p>$f_2 = \frac{2 \times 360}{2 \times 0.4} = 900 \, \text{Hz}$.</p> <p>Therefore, the frequency of the second harmonic is $900 \, \text{Hz}$.</p>