My book derives the result of what the propagation speed of a wave on a string is: v = √(F/mu) It did it using the wave equation, so I wonna ask something about this equation. My book started on waves by describing waves that are harmonic, i.e. inwards force line proportional to the the displacement from the equilibrium. Therefrom they derived the wave equation. But I'm pretty sure, that the wave equation is more general and provides a more deep description. Isn't it so? I mean, isn't it applicable for all kinds of waves? They then derived the result for v by showing that the string can be described by the wave equation. Does this then mean, that the inwards force is indeed proportional to the displacement from the equilibrium line on a string?