What Is a Fluid?

In physics, a fluid is a substance that continually deforms (flows) under an applied shear stress. Fluids are a subset of the phase of matter and include liquids, gases, plasmas and, to some extent, plastic solids.

Fluids easily move when pressure is applied. Their lack of shape allows them to flow.

Fluids have the following 2 characteristics:

1. not resisting deformation, or resisting it only slightly (viscosity),
2. the ability to flow (also described as the ability to take on the shape of the container)

In the realm of physics, any substance which can flow is called a fluid. Fluids are generally liquids, gases and plasma.

The study of fluids is divided into two branches, namely $Fluid$ $dynamics$ and $Fluid$ $statics$

Fluid dynamics deals with the study of flowing fluids.

Fluid statics deals with the study of fluids at rest.

a substance, as a liquid or gas, that is capable of flowing and that changes its shape at a steady rate when acted upon by a force tending to change its shape.

Two types of fluids exist -- Newtonian fluids and non-newtonian fluids. The latter refer to fluids that do not obey Newton's Laws of motion.

The former can be modelled with the Navier Stokes Equations that are essentially Newton's Laws.

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