Stokes Drag Force — the NEET Physics formula with its derivation, variables, validity constraints and worked solver.
Stokes' Drag Force This formula is valid for calculating the viscous drag force on a small, spherical object moving slowly through a fluid under laminar flow conditions (low Reynolds number). Stokes' Law is derived by considering the shear stress exerted by the fluid on the surface of the sphere. The shear stress ( ) is proportional to the velocity gradient ( dv dy ). Integrating the shear stress over the entire surface area of the sphere yields the total drag force. The resulting proportionality constant is 6 . The flow must be laminar (low Reynolds number). The object must be spherical for this simplified formula to hold. Viscosity ( ) must be constant over the temperature range. Assuming the drag force is always proportional to v 2 (this is true at high Reynolds numbers, but not for Stokes' regime). Confusing the fluid's density ( ) with the fluid's viscosity ( ). Applying this formula to non-spherical objects (e.g., cylinders or plates).