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Fluid mechanics is branch of physics that deals with the mechanics of fluids in various states and the reaction of the forces acting upon them. The state could be liquid, gaseous or plasma state. Liquids and gases have many properties in common because of their common ability to flow. A plasma is also a fluid and can be described by the many principles of fluid mechanics and it also has electromagnetic properties.

Did you know: The study of liquids in motion is called hydrodynamics and the study of gases in motion is called aerodynamics.

Fluid mechanics can be divided into two

• Fluid statics / hydrostatic  - study of fluids at rest
• Fluid dynamics / hydrodynamics  - study of the effects of forces on the fluids in motion

Fluid statics
Fluid statics is study of the conditions in which fluids are at rest and in stable equilibrium state. It offers physical explanations for the many phenomenon we observe in everyday life. A few of them being why the atmospheric pressure changes with altitude, why wood floats on water and why the surface of the water is flat irrespective of the container.

A fluid at rest does not go through any shear stress and only experiences influences of the surrounding liquids which is in the form of pressure. This equilibrium condition of the fluid is called hydrostatic condition.

The major concepts in fluid statics are stress, pressure and density. Shear stress is a stress state where the stress is parallel to the surface of the fluid. In normal stress, the stress is perpendicular to the surface of the fluid. Liquids pushing against the walls of the container represent tiny bits of force and all of this force results in pressure. Density is mass per unit volume. Each fluid has density which can be determined experimentally. When liquids of different densities are mixed you might have observed that the liquids settle in layers. This is because the liquids settle into layers based on their density. The liquid with the highest density settles at the bottom and the liquid with the lowest density settles on the top.

Fluid dynamics
Fluid dynamics deals with the fluid flow of liquids and gases. It involves calculating the various properties of fluid such as velocity, pressure, density and temperature as functions of space and time. It has a wide range of applications such as calculating forces and moments on aircrafts, determining the mass flow rate of petroleum through pipelines and predicting the evolving weather pattern.

The important concept in fluid dynamics is flow. The flow of a liquid is classified into different ways based on the property of flow. If the movement of the liquid does not change over time it is called a steady flow. If the flow itself has properties that change over time then it is called unsteady flow. A smooth flow of liquid is called laminar flow. A flow that has chaotic and nonlinear motion is said to have turbulent flow.

Gases can be treated as compressible liquids because the volume that contains them can be reduced. Liquids can also be compressed but there are limitations on the amount of compression. Therefore liquids are considered incompressible.

Fluid dynamic is used in the field of oceanography, meteorology, climate science, aeronautics, geology and geophysics, plasma physics etcl.

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