By Newton’s law of viscosity,

where \(\frac{F}{A}\) is the viscous drag per unit area, \(\frac{dv}{dy}\) is the velocity gradient and η is the coefficient of viscosity of the fluid. Rewriting the above equation as

**SI unit :** the pascal second (abbreviated Pa.s), 1 Pa.s = 1 N.m^{-2} .s

CGS unit: dyne.cm^{-2} .s, called the poise [symbol P, named after Jean Louis Marie Poiseuille (1799 -1869), French physician].

[Note : Thè most commonly used submultiples are the millipascalsecond (mPa.s) and the centipoise (cP). 1 mPa.s = 1 cP.]