Lagrangian formulation of Newtonian mechanics
Newtonian mechanics describes the working of almost everything around us. But at certain times we have to use additional tools to make our understanding better.
Lagrange developed a way to determine the basic results and more advanced results of mechanics using a modified version of Newtonian mechanics called the lagrangian formulation of the same.
This mainly depends upon the concept of energy. Here is a very general example of its use.