What is fluid??.

Fluid :- Fluid is a sustance which is capable of flowing and deforms continuously under the action of shearing stress.
  
  Fluid may be divided into three types:-
a. Liquid.
B. Vapours.
C. Gases.
Liquid : Liquids offer great resistance to compression and are not much affected by chance of temperature. lt is practically incompressible. Liquids can take the shape of the container.
Vapours: vapours are highly compressible  any change in temperature and pressure will change their density(mass/volume). We can also  condensate vapours by condensation(it is a process when vapours comes into liquid form with the help of cold medium).
Gas : It is also highly compressible and also change its density when temperature and pressure will change. In gases the mean distance between the molecules is very large as compare to liquid and solid and the intermolecular forces of cohesion are less.
Fluids can further classified as;
1. Ideal fluid
2. Real fluid
Ideal fluid :- It is one that is incompressible and has no viscosity and surface tension. Students ideal things don't exist in nature but it is useful for consider a process, calculation, etc. It is also called perfect and non viscous fluid.
Real fluid :- A fluid which posses surface tension, viscosity and compressibility is known as real fluid, all fluids are called real fluid.Real fluid offer certion amount of resistance during fluid motion.
Real fluid can also classified as :
1. Newtonian fluids.
2. Non newtonian fluids.
Newtonian fluids: fluids which obey Newton's law of viscosity is called newtonian fluids. 


Examples : Water, oil, gasoline, alcohol, etc.
Mathematically    τ ∝  du/dy
τ= Shear stress.
du/dy = Velocity gradient.
ᵤ = Coefficient of viscosity.
The slope of the straight line for newtonian fluid gives the viscosity(). Gases and thin liquids obey Newton's law of viscosity so these fluids behave like newtonian fluids.
Non newtonian : fluids fluids which doesn't obey Newton's law of viscosity is called non newtonian fluids.
Examples of non-Newtonian fluids are slurries, suspensions, gels and colloids.

  Some other fluids are thixotropic fluids, and ideal plastics fluid.

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