Stokes friction force $$\boldsymbol{F}_{ \text R}$$ describes the friction of a spherical paricle with radius $$r$$ moving in a liquid having viscosity $$\eta$$. The paticle moves with velocity $$v$$ in this fluid. The Stokes friction force $$\boldsymbol{F}_{ \text R}$$ acts anti-parallel to the velocity $$\boldsymbol{v}$$. The velocity may be caused, for example, by the gravitational force acting on the particle.