Past exam of the mathematics course of the University of Cambridge / 2023 / ib / Paper 1 / 16C / a / i / Solution

Across the two vertical faces of a control rectangle, the mass fluxes arewith opposite signs, since $u$ is independent of $x$. There is no flux across the horizontal faces because the $y$-velocity is zero. The net mass flux is therefore zero, equivalently

Solved by gpt-5.6-sol high.