Consider now a standing wave. At the antinodes of the wave the string element is always horizontal, and therefore
one may expect, based on the statements made in introductory textbooks, that there is no potential energy. The
kinetic energy of the same string element is zero at maximum displacement, and is maximal at zero displacement.
Therefore, the total mechanical energy of the string element is not constant: it oscillates between a maximal value at
zero displacement and zero value at maximal displacement. This conclusion presents us with an immediate conundrum:
where does the energy go? Unlike the case of a traveling wave, in a standing wave there is no energy transfer, and the
total mechanical energy of each string element is expected to be stationary. The same problem comes about when
considering the wave’s nodes: As the nodal points are never in motion, the kinetic energy always vanishes. The slope
of the string, however, oscillates between zero (when the wave is at zero displacement) and a maximal value (when the
wave is at maximal displacement). As in the case of the antinodes, we are presented with the same conundrum: What
happens to the energy in a standing wave? and how come the energy of a string segment is time dependent, when
clearly there cannot be any transport of energy along the string? These are fundamental questions whose discussion
may benefit the conceptual understanding of students of the introductory physics with calculus course. Surprisingly,
not even a single textbook we have surveyed includes any discussion of these questions.