Originally Posted by

**TriKri** Hi!

Can someone please explain to me: What does Bernoulli's principle really mean? I don't get the formula.

First of all, velocity has a direction. And second velocity is relative. Let me explain what I mean: Bernoulli's principle says that

$\displaystyle \frac{v^2}{2}+gh+\frac{p}{\rho}=\text{constant}$

if we derive this by some variable x, we get:

$\displaystyle v\frac{\delta v}{\delta x}+g\frac{\delta h}{\delta x}+\frac{1}{\rho}\cdot\frac{\delta p}{\delta x}=0$

This would mean that if the velocity is high from the beginning, the value of $\displaystyle \frac{\delta v}{\delta x}$ is of big importance to the equation, but if the velocity is zero, the value of $\displaystyle \frac{\delta v}{\delta x}$ doesn't matter at all? Can a liquid be "still"? In that case, in relation to what is the velocity of the liquid measured?