De wet van Stokes

Consider an object moving through a fluid. The fluid layers near the object get dragged along with it and experience relative velocity, resulting in a viscous force on the object. The viscous force depends on the object's shape, size, speed, and fluid properties. 

For a spherical body moving inside a fluid, the viscous force is proportional to the sphere's radius, speed, and fluid viscosity.

Solving the equation dimensionally, gives the dependence of viscous force on these parameters. The expression obtained is Stokes' law, indicating that the viscous force is directly proportional to the object's velocity.

If a sphere is dropped through a fluid, the object's weight pulls it downward, while the viscous and buoyant forces act upward.

As the velocity increases due to acceleration, the viscous force increases. When the viscous force balances the weight and buoyancy, the sphere falls with a constant velocity, called terminal velocity.

For a highly viscous liquid, the terminal velocity can be measured experimentally. Then, using Stokes' law, the viscosity can be obtained.