In Stokes' law, the drag force on a sphere of radius r moving at velocity v is F = 6πηrv. This gives terminal velocity ∝ r². If two spheres have radii in ratio 2:1, their terminal velocities are in ratio:

Since the same volume of fluid must pass every cross-section per second (continuity), the fluid speeds up where the pipe narrows; Bernoulli's equation then says this faster-moving fluid has lower pressure - the principle behind a venturi meter and an aircraft wing's lift.