连续性方程

There are several equations of continuity. However, here we are concerned about fluids, so we will provide you with a simple equation of continuity, which seems quite remarkably short. And it is remarkable!

It states the following. Say that we are flowing a certain path (e.g. a tube) where the cross-sectional area, at a given point, is $A$ and the velocity of the fluid at that point is $v$, then $Av$ is the same as it is at another point; i.e. $A(t)\cdot v(t)$, for all times $t$ at all locations in the tube. See the practice section of this topic for examples.