Pressure

Pressure measures how concentrated a force is over a given area.

Key Definition Pressure is defined as the force acting per unit area on a surface, and is calculated using the equation:

p = F / A

where p is the pressure in pascals (Pa), F is the force in newtons (N), and A is the area in metres squared (m²). One pascal is equal to one newton per square metre.

Pressure is directly proportional to force and inversely proportional to area, which explains why sharp objects like knife blades create high pressures, while wide objects like snowshoes create low pressures.

Everyday examples of pressure rely on changing the area over which a given force acts — designers increase the area to reduce the pressure, or decrease the area to increase it.

• Beneath the surface of a liquid, pressure increases with depth, because a deeper point has a greater weight of liquid above it.
• Pressure in a liquid also increases with the density of the liquid, because a denser liquid has more mass per unit volume, so the weight above any point is larger.
• Pressure at a given depth acts equally in all directions.

Extended Extended students can calculate the pressure change beneath a liquid surface using the equation:

Δp = ρgΔh

where Δp is the change in pressure (Pa), ρ is the density of the liquid (kg/m³), g is the gravitational field strength (N/kg), and Δh is the change in depth (m).

This equation shows that the pressure change is directly proportional to both depth and density — doubling either quantity doubles the pressure change.

The total pressure at a depth below a liquid surface equals atmospheric pressure plus the pressure due to the liquid column.