Stokes' law applies to particles that are doing what?

• A. Settling Only
• B. Rising Only
• C. Settling and Rising
• D. Dissolving

Answer: (C)

Stokes' law describes the viscous drag on a small spherical particle moving slowly through a viscous fluid in laminar (creeping) flow. The drag force is proportional to the velocity, and at terminal velocity the drag balances the net gravitational force (weight minus buoyancy). This means a particle can settle downward if it is denser than the fluid, or rise upward if it is less dense than the fluid. Dissolving, on the other hand, involves diffusion and mass transfer, not motion of a solid particle through the fluid with drag, so it’s not described by Stokes’ law. Therefore, Stokes' law applies to particles that are settling or rising.