I have been thinking about this a lot. It is about conservation of energy. A simplified analysis using Bernoulli's Principle for one-dimensional incompressible liquid fluids should be good enough to gain some understanding (at least for me). Let us also neglect gravitational effects. Assume you have a pressurized pipe filled with liquid plugged by a spring loaded valve at one end. What is the (fluid) force on the valve? I understand that when the valve is CLOSED the force would simply be F = P*A, where P is the STATIC pressure and A is the Area on the valve. But what if you OPEN the valve a little so that liquid begins to flow? Is the force on the valve determined by the STATIC pressure only, or is it determined by the TOTAL pressure, i.e. including the kinetic part of the fluid motion? What is the Force? Please help.