Pressure is the magnitude of the perpendicular force component exerted per unit area, over a given surface:
That word "perpendicular" is doing real work. If a force pushes at an angle against a surface, only the component pushing straight into the surface contributes to pressure — the component sliding along the surface doesn't count.
A 720 N crate rests on a square base 0.60 m on each side. Find the pressure the crate exerts on the floor.
This connects straight back to Lesson 8.1's ideal fluid model: the volume and density of a given amount of an incompressible fluid stay constant, no matter how much pressure is exerted on it.
A solid block resting on a table exerts pressure through one clean, well-defined force. A fluid is messier — and more interesting. The pressure a fluid exerts on a surface is the net result of countless individual particle collisions against that surface, all happening constantly and from every direction.
At any point in a fluid, the absolute pressure is the sum of a reference pressure P₀ (usually atmospheric pressure) and the gauge pressure — the extra pressure contributed by the fluid itself:
For a vertical column of fluid, gauge pressure depends on exactly three things — the fluid's density, gravity, and how deep the point is:
Deeper point, denser fluid, or stronger gravity — any of the three increases gauge pressure. Combine the two equations and you get the full picture: P = P₀ + ρgh.
Drag depth and pick a fluid. Watch absolute pressure climb — and watch the particle collisions below speed up as pressure increases.
Higher pressure means more frequent, harder collisions — that's what pressure actually is at the particle level.
A diver is swimming 3.0 m below the surface of a freshwater lake (ρ = 1000 kg/m³). Atmospheric pressure at the surface is P₀ = 1.01×10⁵ Pa. Find the absolute pressure at the diver's depth. (g = 9.8 m/s²)