The system consists of the volume of fluid, initially between the cross-sections A 1 and A 2. In the time interval Δ t fluid elements initially at the inflow cross-section A 1 move over a distance s 1 = v 1 Δ t, while at the outflow cross-section the fluid moves away from cross-section A 2 over a distance s 2 = v 2 Δ t. The displaced fluid volumes at the inflow and outflow are respectively A 1 s 1 and A 2 s 2. The associated displaced fluid masses are – when ρ is the fluid's mass density – equal to density times volume, so ρA 1 s 1 and ρA 2 s 2. By mass conservation, these two masses displaced in the time interval Δ t have to be equal, and this displaced mass is denoted by Δ m : ρ ρ A 1 s 1 = ρ ρ A 1 v 1 Δ Δ t = Δ Δ m , ρ ρ A 2 s 2 = ρ ρ A 2 v 2 Δ Δ t = Δ Δ m .