🫧 Viscosity & Terminal Velocity

📖 The Physics

Why does a marble sink slowly through honey but quickly through water? Viscosity is a fluid's internal friction — its resistance to flowing. A sphere sinking through a fluid feels three forces: its weight down, buoyancy up, and a viscous drag up that grows with speed.

At first the sphere accelerates, but as it speeds up the drag grows until drag + buoyancy exactly balance the weight. Then it stops accelerating and falls at a steady terminal velocity.

Thicker fluid (bigger η) → slower fall. A bigger or denser sphere falls faster. This is exactly the falling-ball method used in the lab to measure a liquid's viscosity.