Viscosity is the measure of the fluid resistance to deformation. When fluids move they tend to deform continuously. This is due to some internal forces (or better stresses) that we can call shear stresses. For some fluids, the deformation is proportional to the magnitude of the shear stresses. These fluids are known as 'Newtonian Fluids'. The SI units of viscosity are kg / (m s) or (Pa s).
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