Since the azimuthal angle φ does not appear in the Hamiltonian, its conjugate momentum p φ is a constant of the motion. This conjugate momentum is the magnitude L of the angular momentum, as shown by the Hamiltonian equation of motion for φ d φ φ d t = ∂ ∂ H ∂ ∂ p φ φ = p φ φ m r 2 = L m r 2