A pressure washer converts the pump's static pressure into the kinetic energy of a fast water jet. Cleaning happens when the jet's impact pressure on the dirt film exceeds the force holding that grime to the surface.

Dynamic pressure

By Bernoulli's equation, exit velocity is v = √(2P/ρ), so a garden hose at a few PSI barely dribbles while a washer at 2,000+ PSI fires water at highway speed. Impact force scales with v², i.e. with pressure itself — doubling PSI roughly doubles the cleaning force at the nozzle face.

Why nozzle angle matters

A 0°/15° pencil jet keeps nearly all the flow concentrated in a tiny spot, so the same PSI delivers far more force per unit area than a 40°/65° fan pattern, which spreads the same flow over a much wider footprint. Narrow tips strip stubborn stains fast but are the ones most likely to gouge a surface; wide fan tips are the safe, general-purpose choice.

Why standoff distance matters

As the jet travels from the nozzle it keeps expanding, so its footprint area grows roughly with the square of the distance — impact pressure falls off in a similarly inverse-square-ish way. Move the wand closer and the same pump pressure gets concentrated onto a smaller patch, multiplying both cleaning power and the risk of surface damage.

Two thresholds, one jet

Every surface has a clean threshold (impact pressure needed to shear grime loose) and a higher damage threshold (impact pressure that starts eroding the material itself). Manufacturers publish per-surface PSI guidance for exactly this reason:

In this simulator the same numbers drive both the grime removal and the visible etching — dwell time matters too, so lingering in one spot at high pressure and close range is the fastest way to damage a surface, even below the instantaneous threshold.