In this fluctuation, the motion form and energy of wave source are driven by one point in the medium to spread by another adjacent point in turn. For the energy of each element, it is not conserved.

When the mass element reaches the equilibrium position, the kinetic energy is the largest, and at the same time it is the largest in stretching or deformation. Therefore, the corresponding deformation potential energy is the largest (for an image point, it is to look at the slope of that point. The greater the slope, the greater the deformation).

At the peak or trough, the slope of the point is zero, its kinetic energy is zero, and the deformation is also zero, that is, the deformation potential energy is zero.