Physics

For standing waves on a string fixed at both ends, the key insight is that the string length must fit an exact number of half-wavelengths. The third harmonic means three half-wavelengths fit along the string, so you can find the wavelength directly from the geometry, then use the wave equation $v = f\lambda$ to unlock the frequency.

For standing waves on a string fixed at both ends, the key insight is that the string length must fit an exact number of half-wavelengths. The third harmonic means three half-wavelengths fit along the string, so you can find the wavelength directly from the geometry, then use the wave equation $v = f\lambda$ to unlock the frequency.