Viscosity & Fluid Resistance — A/L Physics

What must I remember about Viscosity & Fluid Resistance?

• Viscosity Concept: Internal molecular friction of a fluid that resists relative motion between adjacent shearing layers; Dynamic Viscosity coefficient η defined by Newton's Law of Viscosity: F / A = η (dv/dx) (units Pa·s = N·s·m⁻² = kg·m⁻¹·s⁻¹).
• Stokes' Law: Viscous resistive drag force on a smooth spherical particle of radius r moving at low speed v in an infinite medium of viscosity η: F_d = 6 π η r v.
• Terminal Velocity (v_t): Steady maximum velocity attained when gravitational force W is exactly balanced by the sum of upward buoyant upthrust U and Stokes' drag F_d: W = U + F_d => v_t = [ 2 r² (ρ_sphere - ρ_liquid) g ] / (9 η).
• Poiseuille's Law: Steady laminar volume flow rate Q through horizontal capillary tube of radius r and length L under pressure difference ΔP: Q = (π r⁴ ΔP) / (8 η L).

Test yourself (1 minute)

1. Terminal velocity dependence on radius

   Terminal velocity is strictly proportional to the square of radius (v_t ∝ r²); large raindrops fall significantly faster than tiny mist droplets.

2. Boundary wall effect in Stokes' experiment

   Stokes' law assumes an infinitely wide fluid container; in narrow laboratory cylinders, container walls increase drag, requiring Faxen correction.

3. Temperature sensitivity

   Viscosity of liquids decreases rapidly with rising temperature (loosened cohesive intermolecular forces), whereas gas viscosity increases with temperature.

Practise Viscosity & Fluid Resistance questions today

Source: NIE G.C.E. (A/L) Physics Resource Book - Unit 10: Mechanical Properties of Matter

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Where does Viscosity & Fluid Resistance fit in the syllabus?

Unit 10.3 · Year 2 · Term 3