Viscosity & Fluid Resistance — A/L Physics

Viscosity A measure of a fluid’s internal resistance to flow. Viscosity arises from intermolecular forces and momentum transfer between layers of fluid moving at different speeds. A high-viscosity fluid (e.g., honey) flows slowly; a low-viscosity fluid (e.g., water) flows easily. In liquids, viscosity decreases with increasing temperature; in gases, it increases.

Newton’s Law of Viscosity The shear stress between adjacent fluid layers is proportional to the velocity gradient perpendicular to the flow direction. Mathematically, τ = η (dv)/(dy) , where τ is shear stress (force per unit area), η is dynamic viscosity, and (dv)/(dy) is the velocity gradient. This law defines a Newtonian fluid (e.g., water, air) where η is constant at a given temperature. Non-Newtonian fluids (e.g., ketchup, blood) do not obey this linear relation.