Source: High school physics (Chinese)
Problem Sets:
Problem
Two anglers A and B each sit in a boat on a lake, with the boats $24$ m apart. A train of water waves travels across the surface, making each boat bob up and down 20 times per minute. At one instant boat A is at a crest while boat B is at a trough, and at that moment there is exactly one additional crest between the two boats.
Each boat completes 20 oscillations per minute, so the period is $T = \dfrac{60}{20} = 3$ s. With A at a crest, B at a trough, and one more crest between them, the boats span one full wavelength (crest to the next crest) plus a half wavelength (that crest to the trough): $\dfrac{3}{2}\lambda = 24$ m, giving $\lambda = 16$ m. The wave speed is $v = \dfrac{\lambda}{T} = \dfrac{16}{3} \approx 5.3$ m/s.