Fluid

Fluid:

Any substance that can flow is a fluid. A fluid is a substance that deforms continually under the action of an external force. Fluid is a phase of matter that includes liquids, gases and plasmas. Fluids are substances which cannot resist any shear force applied to them i.e. their shear modulus is zero. Air, water, flour dough, toothpaste, molten lava, etc. are all fluids. 

An ideal fluid has the following properties:

1. It is incompressible: its density is constant.

2. Its flow is irrotational: its flow is smooth, there are no turbulences in the flow.

3. It is non viscous: there is no internal friction in the flow, i.e., the fluid has no viscosity.

4. Its flow is steady: its velocity at each point is constant in time. 

Solids oppose the shear stress either by developing a restoring force, which means that the deformations are reversible, or they require a certain initial stress before they deform and start flowing. Ideal fluids, on the other hand, can only be subjected to normal, compressive stress (called pressure). Most fluids offer a very weak resistance to deformation. Real fluids display viscosity and so are capable of being subjected to low levels of shear stress.

Properties of Fluids:

1. They do not oppose deformation, they get permanently deformed.

2. They have the ability to flow. 

3. They have the ability to take the shape of the container. 

The branch of physics which deals with the properties of fluids at rest is called hydrostatics.