The resistance of a fluid (liquid or gas) to a change in shape, or movement of neighbouring portions relative to one another is called ______. 

Key Points

Viscosity:

• When a fluid moves relative to a solid or when two fluids move relative to each other. The property that represents the internal resistance of a fluid to motion (i.e. fluidity) is called as viscosity.
• The fluids for which the rate of deformation is proportional to the shear stress are called Newtonian fluids and the linear relationship for a one-dimensional system is shown in the figure. The shear stress(τ ) is then expressed as,​

          \({\rm{\tau }} = {\rm{\mu }}\frac{{{\rm{du}}}}{{{\rm{dy}}}}\) ;

• where  \( \frac{{{\rm{du}}}}{{{\rm{dy}}}}\) is the shear strain rate and µ is the dynamic (or absolute) viscosity of the fluid.
• The dynamic viscosity has the dimension ML-1T-1 and the unit of kg/m.s or, \((N-sec)/m^2 \) or Pa-s . A common unit of dynamic viscosity is Poise which is equivalent to 0.1 Pa.s.

​ Additional Information

• Kinematic viscosity (ν):The ratio of dynamic viscosity (µ ) to density (ρ) . 
•  \(υ =μ/ρ\) ;  It has got the dimension of L2T-1 and unit of stoke (1 stoke = 0.0001 m2/s).
• Fluidity: The ability of a substance to flow easily.
• Specific weight(ω):  weight per unit volume of the substance is called specific weight. (ω = ρg)
• Turbidity: Turbidity is the cloudiness or haziness of a fluid caused by individual particles (suspended solids) that are generally invisible to the naked eye, similar to smoke in air. The measurement of turbidity is a key test of water quality.