For time-invariant system, the action S relates simply to the abbreviated action W on the stationary path as Δ Δ S = Δ Δ W − − E Δ Δ T for energy E and time difference Δ Δ T = t 2 − − t 1 . For a rigid body with no net force, the actions are identical, and the variational principles become equivalent to Fermat's principle of least time: δ δ ( t 2 − − t 1 ) = 0.