[[Fluids]]
Pressure is perpendicular force acting on a surface divided by area.

# Summary
* $P=\frac{F_\perp}{A}$
* Scalar.
* Incompressible fluids keep constant density and volume when pressure changes.
* $P=P_0+pgh$
* At the fluid surface h is 0.
* Measured in pascals (1 Pa = N/m^2)
# Absolute vs Gauge Pressure
Absolute pressure is total pressure: $P=P_0+pgh$, where $P_0$ is a reference pressure such as atmospheric pressure ($P_{atm}$) and gauge pressures is $pgh$. Positive gauge pressure means above atmospheric, negative means below.