viscosity

Meaning: Viscosity (η) measures how strongly a fluid resists flow because adjacent layers moving at different speeds rub against each other. Honey has high η; water has low η. In a real fluid, viscous friction dissipates mechanical energy, so pressure falls along a horizontal tube of constant radius. An ideal fluid is treated as having zero viscosity, the assumption behind Bernoulli's equation. For liquids, η generally decreases as temperature rises; for gases it generally increases.

Equation: Q = πr⁴ΔP/(8ηL), where Q is volume flow rate (m³/s), r is tube radius (m), ΔP is the pressure difference along the tube (Pa), η is viscosity (Pa·s), and L is tube length (m).

On the MCAT: Passages ask how flow rate changes when r, L or η changes, often for blood in vessels. Because Q depends on r⁴, halving the radius at fixed ΔP cuts Q by a factor of 16. Another setup is a sphere falling slowly through a fluid: the drag force 6πηrv (r is sphere radius in m, v is speed in m/s) grows with speed until it balances the other forces, giving terminal velocity.

Common confusion: Viscosity is not density. Oil is less dense than water yet more viscous, and it is the fluid's property, not the tube's.