Variational principle

• How Does Hamilton’s Principle Work with Semiholonomic Constraints?
  For nonintegrable velocity constraints, distinguish Hamilton’s principle on constrained paths from the d’Alembert variation used for ideal constraints.
• How Is the Lagrangian Derived in Classical Mechanics?
  Derive the Euler–Lagrange equations from the stationary-action principle, see why L = T − V works, and follow a worked spring example.