Lagrangian

The Lagrangian L of a system is simply the kinetic energy minus the potential energy:

L = T - V

This might seem like an uninteresting quantity, but it is the basis of much of classical mechanics. The Euler-Lagrange equation is

∂L/ ∂q = d/dt (∂ L/∂ q′ )

where q is known as a generalized position variable (such as the Cartesian coordinate x) and q′ is its time derivative.