Newton's Law of Viscosity (F = ηA(v/l) or η = (F/A)/(v/l))
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F = ηA(v/l) or η = (F/A)/(v/l)
What each symbol means
| Symbol | What it stands for |
|---|---|
| A | Area of the fluid layer (m²) |
| F | Viscous force between fluid layers (N) |
| l | Distance between the layers (m) |
| v | Relative velocity between the layers (m/s) |
| η | Coefficient of viscosity (Pa·s or N·s/m² or Pl) |
| v/l | Velocity gradient or strain rate (s⁻¹) |
When to use this
Defines coefficient of viscosity as ratio of shearing stress to strain rate. Unlike solids, stress depends on rate of strain, not strain itself. SI unit: poiseuille (Pl) = Pa·s = N·s/m². Viscosity of liquids decreases with temperature; viscosity of gases increases with temperature.