I have finished writing sections 2.1 — The Lagrangian and 2.2 — The Principle of Stationary Action for Lagrangian mechanics.

Section 2.1 gives an introduction to the Lagrangian formalism, and section 2.2 describes how the equations of motion of a system can be derived from an incredibly elegant approach. I cannot stress how important section 2.2 is for any aspiring physicist. The principle of stationary action is at the heart of pretty much every topic in theoretical physics!