1. The problem statement, all variables and given/known data https://ia701205.us.archive.org/11/items/Mechanics_541/LandauLifshitz-Mechanics.pdf [Broken] Page 72 of the book itself, but Page 81 of the PDF. Problem 1. 2. Relevant equations See Section 24 of the book. 3. The attempt at a solution So far I have completed and understand the kinetic energy part of both the transverse and longitudinal modes of oscillation of the molecules for Problem 1, as well as the potential energy for the longitudinal mode. For some reason I cannot figure out, after many hours of tedious thinking why they have the potential energy they do for the transverse case. If anyone could explain this, it would be very helpful.