Hydrostatic equilibrium and fluid motion.

Fluids include gasses and liquids.

- For a gas, the density changes readily, and the molecules are far apart.
- For liquids, molecules are close together, and the density remains basically constant

A fluid is defined as a matter that flows under the influence of external forces.

Fluids cannot maintain a fixed structure, but fit the shape of the container they're confined to.

# Pressure

Pressure is defined as **force *per* unit area*** exerted by a fluid.

$$ p = \frac{F (N)}{A(m^2)} $$
The SI unit for pressure is called a pascal ($Pa$), equal to $\frac{N}{m^2}$.

Pressure is exerted on the fluid's container. 

Pressure is the same everywhere at the same depth.

$$ 1atm = 101.3 kPa = 14.7 psi $$