https://www.youtube.com/watch?v=jCD_4mqu4Os&list=PLTjLwQcqQzNKzSAxJxKpmOtAriFS5wWy4
00:00 Why all this?
00:52 Action Functional
01:53 Nature is extemal
02:44 Calculate the action
03:52 Structure of the Euler-Lagrange equation
05:20 Lagrange function
05:54 How to use Euler-Lagrange equation
More: https://en.fufaev.org/euler-lagrange-equation
Books by Alexander Fufaev:
1) Equations of Physics: Solve EVERY Physics Problem
https://en.fufaev.org/physics-equations-book
2) Alexander Fufaev and His Never Ending Story:
https://en.fufaev.org/lifestory