force of drag

Meaning: Drag comes from fluid viscosity and pressure differences around a moving object. It opposes motion relative to the fluid and grows as speed increases. A falling object therefore accelerates less and less until drag plus the buoyant force equals its weight. Net force is then zero and speed stays constant (terminal velocity).

Equation: Stokes' law for a small sphere moving slowly through a viscous fluid: Fdrag = 6πηrv, where Fdrag is in N, η is viscosity in Pa·s, r is the sphere's radius in m, and v is speed relative to the fluid in m/s. At higher speeds drag is often modeled as Fdrag = ½CρAv², where C is a dimensionless drag coefficient, ρ is fluid density in kg/m³, A is cross-sectional area in m², and v is in m/s.

On the MCAT: Typical setups are a sphere falling in a viscous liquid, a skydiver or parachute, or sedimentation and centrifugation. Expect to write mg = Fdrag + Fbuoyant at terminal velocity, predict that higher viscosity lowers terminal speed, or read a velocity-time graph that curves up toward a horizontal asymptote while acceleration falls to zero.

Common confusion: At terminal velocity, drag does not vanish and the object is not at rest. Drag is balanced by the other forces, so net force is zero and velocity is constant. Drag also differs from kinetic friction between solids, which does not depend on speed and scales with the normal force.