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How do hydrostatics and hydrodynamics differ?

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A diagram of a cylindrical vessel containing gas, with a movable piston held down by a weight. Countless small dots beneath the piston represent gas particles in rapid, random motion, illustrating Bernoulli's model of gas pressure. — physics As conceived by Daniel Bernoulli in *Hydrodynamica* (1738), gases consist of numerous particles in rapid random motion. He assumed that the pressure of a gas is produced by the direct impact of the particles on the walls of the container.

In the field of physics, hydrostatics and hydrodynamics are distinct branches of fluid mechanics that categorize the study of fluids based on their state of motion.1

Hydrostatics is the science of fluids at rest. It focuses on the characteristics of fluids in a stationary state, specifically examining the pressure within a fluid or the pressure exerted by a fluid on an immersed body. A primary application of hydrostatic principles is calculating how pressure changes with depth in liquids or altitude in the atmosphere.12
Hydrodynamics is the science of fluids in motion. It deals with the movement of fluids and the forces acting on solid bodies that are immersed in and moving relative to those fluids. While hydrostatics relies on static pressure, hydrodynamics incorporates dynamic principles—such as viscosity and Bernoulli’s law—to analyze flow patterns, drag forces, and the behavior of fluids through tubes or around obstacles.134

In modern scientific contexts, hydrodynamics is often referred to as fluid dynamics, and the broader study of both stationary and moving fluids is collectively known as fluid mechanics.1