The classical linear relation between shear stress and velocity gradient in a Newtonian fluid (viscosity law) was proposed by which scientist?

Introduction / Context:
For many common fluids (water, air, light oils), the shear stress τ is proportional to the rate of strain (velocity gradient) du/dy. This linear proportionality defines Newtonian behaviour and introduces the key material property called dynamic (absolute) viscosity μ.

Given Data / Assumptions:

• Simple shear between parallel plates is the reference configuration.
• Small enough rates of deformation to remain within Newtonian regime.

Concept / Approach:
Newton’s law of viscosity states τ = μ * (du/dy). Here μ is a constant for a Newtonian fluid at a given temperature and pressure. Non-Newtonian fluids do not follow this linear law (μ depends on shear rate).

Step-by-Step Solution:
Recognize the linear τ–(du/dy) constitutive equation.Attribute the proposal to Isaac Newton, who formalized the proportionality.Identify μ as dynamic (absolute) viscosity with SI units Pa·s (N·s/m^2).

Verification / Alternative check:
Textbooks and rheology references define Newtonian fluids via this relation. Bernoulli contributed to energy principles, Chezy/Bazin to open-channel empirical formulas, Helmholtz to vortex dynamics—not the viscosity law.

Why Other Options Are Wrong:
Bernoulli: energy/pressure–velocity relation, not constitutive law.Chezy and Bazin: channel resistance relations.Helmholtz: vorticity and fluid kinematics.

Common Pitfalls:

• Confusing dynamic viscosity μ with kinematic viscosity ν = μ/ρ.
• Applying Newton’s law to non-Newtonian fluids without checking rheology.

Final Answer:
Newton