‘Toilets flush backwards in Australia’ — because of the Earth’s rotation. The Coriolis force is REAL… but at the scale of a sink it is about ten million times weaker than the swirl left by how the basin was filled and shaped. The Rossby number measures which wins. It only dominates at the scale of hurricanes. Slide the basin size and watch the crossover. Verdict: FLUFF (real force, wrong scale).
The Rossby number Ro = U/(f·L) compares inertial flow to the Coriolis effect (f = 2Ωsinφ ≈ 10⁻⁴ s⁻¹). At a sink (U~0.1 m/s, L~0.3 m) Ro ~ 5000 — Coriolis is utterly negligible next to residual circulation and basin geometry. Only when L reaches ~100 km (Ro ~ 1) does rotation organise the flow, which is why cyclones DO obey it (and spin oppositely by hemisphere). Under lab conditions — a symmetric tank left still for hours — the tiny bias is measurable, but no household drain qualifies. A fail-loud self-check throws unless Ro ≫ 1 at sink scale. ◆ VERDICT: FLUFF — the force is real, the household claim is not.
The threshold (Ro~1 near ~100 km) is order-of-magnitude; the point is exact — Coriolis loses to geometry by orders of magnitude at sink scale. It CAN be shown in a controlled tank, so this is ‘fluff’ (misapplied real physics), not fully ‘debunked’.