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Define a fluid and distinguish between ideal and real fluids.
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Fluid is a substance which is capable of flowing OR Fluid is substance which deforms continuously under the influence of a shear or tangential force. OR The tendency to deform continuously is called fluidity and act of continuous deformation is called as flow.

Classification of fluids-

Ideal Fluid Real Fluid
Fluids which don’t have viscosity and are incompressible are termed as ideal fluid such fluid do not offer shear resistance. Fluids which do posses viscosity are termed as real fluids. These fluids always offer shear resistance.

Real Fluid further classified into two types

1. Newtonian fluid- The fluids which obey Newton’s law of viscosity are called as Newtonian fluids such fluids exhibit linear relationship between shear stress and rate of angular deformation. Eg. - water , air etc.

2. Non-Newtonian Fluid- Fluids which do not follow the linear relation between shear stress and rate of angular deformation are termed as Non Newtonian fluids. Hence for such fluids;

$$τ=constant×\bigg(\dfrac{dy}{dx}\bigg)^n.$$