Pressure

Definition

Pressure ( $P$ ) is the measure of force exerted per unit of area: $P = F/A$ .

It governs hydraulics, fluid flow, and atmospheric behavior.

Density ( $\rho$ ):
Pressure ( $P$ ):
$P = \frac{F}{A}$ (Force / Area)
How to read: “The pressure P is equal to the force F divided by the area A.”
Meaning: Force spread over an area—same force, smaller area means higher pressure.
Unit: Pascal (Pa). $1 \text{ Pa} = 1 \text{ N/m}^2$ .
How to read: “One pascal is equal to one newton per square meter.”
Meaning: SI unit of pressure—one newton of force spread over one square meter.
Pressure in a Liquid: Depends on depth and weight density. It is exerted equally in all directions.
$\text{Pressure} \propto \text{depth}$
How to read: “The pressure is directly proportional to the fluid depth.”
Meaning: Deeper in a liquid, more fluid weight presses down—increasing pressure linearly with depth.
Atmospheric Pressure: Caused by the weight of air. At sea level, it is $\approx 101.3 \text{ kPa}$ .
Boyle’s Law: For a fixed amount of gas at constant temperature, pressure and volume are inversely proportional.
$P_1 V_1 = P_2 V_2$
How to read: “The initial pressure times the initial volume is equal to the final pressure times the final volume.”
Meaning: Boyle’s law—at fixed temperature, pressure and volume are inversely proportional.