Hi, I have a conceptual question regarding Lagrangian dynamics. It has to do with the potential energy formulation. My instructor today mentioned something in class that does not make much sense to me. Here is he most basic example that illustrates my confusion: Take a simple 1dof system of a spring and a mass: A mass M on top of a vertical massless spring (stiffness K), constrained to only move up and down (with and against gravity). I will call the mass's upward displacement from the neutral position +x. Goal: determine the equations of motion using Hamiltonian or Lagrangian methods. The kinetic energy is simple: [tex]KE=.5m(v)^2[/tex] v being the time derivative of x. The Potential energy is where Im confused. Would it be: PE = [tex].5Kx^2 [/tex] OR [tex]PE = .5Kx^2 + m*g*x[/tex] I would guess the latter, but apparently I am wrong. Any advice is appreciated.