The gravity components will generally not be constants, however for most applications either the coordinates are chosen so that the gravity components are constant or else it is assumed that gravity is counteracted by a pressure field (for example, flow in horizontal pipe is treated normally without gravity and without a vertical pressure gradient). The continuity equation is: ∂ ∂ t ρ ρ + 1 r ∂ ∂ r ( ρ ρ r u r ) + 1 r ∂ ∂ φ φ ( ρ ρ u φ φ ) + ∂ ∂ z ( ρ ρ u z ) = 0.