This is a problem related to flowing fluids, a topic referred to as
fluid dynamics. Previously we were dealing with static fluids, a topic referred to as
hydrostatics. In essence, yes, the pressure still varies with height, but in flowing fluids it can now also vary due to changes in the velocity field.
Also note that Bernoulli's principle is simply an application of conservation of energy, with some assumptions. If I remember correctly, one assumption was to neglect heat losses. In that equation, we have (static pressure + dynamic pressure + GPE) at point one is equivalent to (static pressure + dynamic pressure + GPE) at point 2. The author has stated assumptions with have reduced the equation. The
static pressures in this case are the same. The dynamic pressure is given by:
21ρv2It appears from that equation in the figure that the fluid moves because gravitational potential energy is in effect, converted to a moving energy. As for WHY the pressures are the same, think of it first like doing a force diagram, where we gather all the forces first. In the previous example, the local thermodynamic pressure acting on a fluid element was the same, but there was an additional weight of the fluid element to be considered. Similarly, in this case, we first collect all the energy terms and then simplify the balance equation - everything on the left equals everything on the right because in an isolated system, energy is conserved, i.e. it cannot be created or destroyed, it just remains the same, albeit in different forms. In essence, Bernoulli's equation then states that at point 2, the pressure energy + motion energy (=0) + GPE is equivalent to the pressure energy + motion energy + GPE at point 2. Now that we have constructed the full equation, then we simplify.