Newton’s law of viscosity: In a steady flow of a fluid past a solid surface, a velocity profile is set up such that the viscous drag per unit area on a layer is directly proportional to the velocity gradient.
When a fluid flows past a solid surface in a streamline flow or when a solid body moves through a fluid, the force of fluid friction opposing the motion is called the viscous drag. The magnitude of the viscous drag of a fluid is given by Newton’s law of viscosity.
If \(\frac{dv}{dy}\) is the velocity gradient, the viscous drag per unit area on a layer,
\(\frac{F}{A}\) ∝ \(\frac{dv}{dy}\)
∴ \(\frac{F}{A}\) = η\(\frac{dv}{dy}\)
where the constant of proportionality, y, is called the coefficient of viscosity of the fluid.
