Interactive Teaching Tool · Fluid Properties · Stokes' Law

⚫ Falling-Sphere Viscometer

Drop a sphere into a jar of viscous fluid and watch it settle to a steady terminal velocity. From that speed you measure the fluid's viscosity — the classic falling-sphere viscometer. Change the fluid, the temperature, the sphere material and size, and see density, specific gravity, buoyancy and viscosity all in play. Some spheres even float!

Working fluid

Temperature 20 °C

Viscosity falls sharply as the fluid warms.

Sphere material

Sphere diameter 5 mm

Press drop to release from the top.

Fluid

Density ρ—

Specific weight γ—

Specific gravity—

Dynamic visc. μ—

Kinematic visc. ν—

Sphere & motion

Sphere density—

Weight / Buoyancy—

Terminal velocity—

Live velocity—

Reynolds number—

— —

The physics behind the game

A sphere released in a still fluid feels three forces: its weight, the upward buoyancy, and viscous drag. When they balance, it stops accelerating and falls at a constant terminal velocity. For slow (creeping) flow, drag follows Stokes' law and the balance gives:

v_t = (ρ_s − ρ_f) g d² / (18μ) • μ = (ρ_s − ρ_f) g d² / (18 v_t) • Re = ρ_f v_t d / μ

The second form is exactly how a falling-sphere viscometer works: time the sphere over a known distance to get v_t, then solve for μ. If ρ_s < ρ_f the sphere floats instead. Stokes' law is only valid for Re < ~1; for a heavy sphere in a thin fluid (large Re) the true terminal velocity is far lower than the formula predicts, so the tool flags that regime. Values are illustrative teaching approximations.

⚫ Falling-Sphere Viscometer

Challenge

The physics behind the game