• id: JME-09-N10
• title: Viscosity and Newton's Law of Viscous Flow
• tags: viscosity, newton, velocity-gradient

Viscosity is internal friction in a fluid that opposes relative motion between adjacent layers. Newton's law: $F = -\eta A (dv/dy)$, where $\eta$ is the coefficient of dynamic viscosity, $A$ is the area of the layer, and $dv/dy$ is the velocity gradient perpendicular to flow. SI unit of $\eta$: Pa s (= N s m$^{-2}$ = kg m$^{-1}$ s$^{-1}$). CGS unit: Poise (1 Pa s = 10 Poise). Dimensional formula: [M L$^{-1}$ T$^{-1}$]. Kinematic viscosity $\nu = \eta/\rho$ (m$^2$/s). Viscosity of liquids decreases with temperature (molecules move apart); viscosity of gases increases with temperature (more molecular collisions).