# Pressure and area relationship in fluids

1. Oct 16, 2015

### David513

Is it fair to say that you should consider the relationship between pressure and area as a function of whether a fluid is moving or standing still?

In other words, when a fluid is moving and you decrease the area, the pressure goes down because there is more uniform translational motion in a moving fluid, therefore fewer molecules of the fluid collide with each other.

Meanwhile, when a fluid is standing still and you decrease the area, the pressure goes up because there is more random translation motion in a resting fluid, therefore more molecules of the fluid collide with each other.

This seems to defy P=F/A and Bernoulli's equation but makes sense intuitively... help!

2. Oct 16, 2015

### JBA

In a flowing fluid the total pressure = static pressure + dynamic pressure; so, the total pressure does not change nor does the mass flow change from one cross section to the other smaller one. For a static fluid when the mass does not change but the volume is reduced then there is an a change of total pressure.