# Stokes' law

## Description

Stokes’ law is an expression for the frictional force – also called drag force – exerted on spherical objects with very small Reynolds numbers (e.g., very small particles) in a continuous viscous fluid. Stokes’ law makes the following assumptions for the behavior of a particle in a fluid: – Laminar Flow – Spherical particles – Homogeneous (uniform in composition) material – Smooth surfaces – Particles do not interfere with each other.

The force of viscosity on a small sphere moving through a viscous fluid (acting on the interface between the fluid and the particle) is depended on the radius of the spherical object, the particle’s velocity and the dynamic viscosity.

## Variables

F_{d} | frictional force - or "Stokes' drag" - acting on the interface between the fluid and the particle (N) |

π | pi |

μ | dynamic viscosity (N*s/m^{2}) |

R | radius of the spherical object (m) |

v | particle's velocity (m/s) |