We have found that most introductory physics with calculus textbooks can be arranged in four main categories:
Category I books include a discussion of the potential energy being proportional to the square of the spatial derivative
of the displacement, similarly to Eq. (1) (see also below) [1]. Category II books discuss the energy in string waves
based on an analogy with a harmonic oscillator, either a spring or a pendulum [2]. Category III books discuss the
power in the waves, but not the potential energy density itself, and find the power calculating the work done by the
elastic force [3]. Finally, Category IV books ignore the question altogether [4].