1 Draw a picture. Show open surfaces, pistons, boundaries, and other features that affect pressure. Include height and area measurements and fluid densities. Identify the points at which you need to find the pressure.

2 Determine the pressure at surfaces.

• Surface open to the air: $p_0=p_{\text{atmos}}$, usually 1 atm.

• Surface covered by a gas: $p_0=p_{\text{gas}}$.

• Closed surface: $p=F\text{/}A$, where $F$ is the force the surface, such as a piston, exerts on the fluid.

3 Use horizontal lines. Pressure in a connected fluid is the same at any point along a horizontal line.

4 Allow for gauge pressure. Pressure gauges read $p_{\text{g}}=p-1\text{atm}$.

5 Use the hydrostatic pressure equation. $p=p_0+\rho gd$.