Stokes’ law

Definition

Stokes' law describes the drag force experienced by spherical objects moving through a viscous fluid. The formula for Stokes' law is $F_d = 6 \pi \eta r v$, where $F_d$ is the drag force, $\eta$ is the dynamic viscosity of the fluid, $r$ is the radius of the sphere, and $v$ is its velocity.

5 Must Know Facts For Your Next Test

1. Stokes' law applies to small, spherical objects undergoing laminar flow in a viscous fluid.
2. The drag force is directly proportional to the radius of the sphere and its velocity.
3. It assumes low Reynolds numbers ($Re < 1$), meaning inertial forces are negligible compared to viscous forces.
4. Stokes' law can be used to determine the terminal velocity of a particle falling through a fluid.
5. It helps in calculating sedimentation rates in suspensions and plays a role in various industrial processes.

Review Questions

• What conditions must be met for Stokes' law to be applicable?
• Write down the formula for Stokes' law and explain what each variable represents.
• How does increasing the radius of a sphere affect the drag force according to Stokes' law?

Related terms

Viscosity:

A measure of a fluid's resistance to deform under shear stress or tensile stress.

Laminar Flow:

A type of fluid flow where all particles move along smooth paths in layers, with minimal mixing between them.

Reynolds Number:

$Re = \frac{\rho v L}{\eta}$; A dimensionless number used to predict flow patterns in different fluid flow situations.