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InformationUDP datagram service

UDP exchanges addressed messages without adding reliable stream delivery.

Definition InformationDNS names, records and caching

DNS distributes typed resource records through a delegated naming hierarchy.

Definition InformationCombinational digital-circuit model

A combinational output depends on current valid inputs after a bounded settling delay.

Model InformationBoolean gates and truth tables

Logic gates compute Boolean functions with truth values 0 and 1.

Definition InformationClocked storage and sequential state

Sequential circuits retain state, so current inputs alone do not determine their behavior.

Model InformationStored programs and instruction sets

Stored instructions control computation through a specified hardware/software interface.

Model PhysicsDisplacement, distance and average speed

Displacement compares endpoints; distance measures the travelled path.

Model PhysicsConstant-acceleration kinematics

Closed formulas require constant acceleration over the interval.

Model PhysicsInertial reference frames and inertia

Newtonian force balances need an inertial frame or explicit inertial terms.

Model PhysicsCircular motion and centripetal acceleration

A constant speed can still have an acceleration.

Model PhysicsMass, gravitational weight and apparent weight

Mass is in kilograms; gravitational weight is a force.

Model PhysicsNormal forces and tension

Contact geometry determines the force direction.

Model PhysicsFree-body diagrams and system boundaries

List forces on the selected body once.

Model PhysicsStatic and kinetic dry friction

Static friction is a responsive force with an approximate limit.

Model PhysicsFluid drag and terminal speed

Terminal speed depends on the chosen drag model.

Model PhysicsImpulse and momentum transfer

Time-integrated net force changes momentum.

Model PhysicsElastic and inelastic collisions

Momentum conservation and kinetic-energy conservation are different.

Model PhysicsConservative forces and potential energy

Potential-energy differences encode path-independent work.

Model PhysicsMechanical-energy conservation and dissipation

Mechanical energy is only part of the total energy.

Model PhysicsHooke’s spring model and elastic energy

A linear spring restores displacement toward equilibrium.

Model PhysicsMechanical power

Power measures the rate of work or energy transfer.

Model PhysicsRigid-body rotational kinetic energy

Rotation stores kinetic energy determined by mass distribution.

Model PhysicsRolling without slipping

No slip constrains velocity at the contact point.

Model PhysicsStatic equilibrium and torque balance

Zero net force alone does not prevent rotation.

Model Physics · Earth & cosmosYoung, shear and bulk elastic moduli

Stress and strain need a loading geometry and a constitutive range.

Model PhysicsDensity and fluid pressure

Density is mass per volume; pressure is normal force per area.

Model PhysicsAbsolute, gauge and differential pressure

A zero gauge reading need not be a vacuum.

Model PhysicsPascal’s principle and ideal hydraulics

A pressure change is transmitted; pressure need not be equal at different heights.

Model PhysicsNewtonian viscosity and Poiseuille flow

Viscosity relates shear stress to velocity gradient.

Model Physics · ChemistrySurface tension and capillary rise

Interface forces can support pressure jumps and capillary columns.

ModelSource checked

Newtonian viscosity and Poiseuille flow

Physics

Viscosity relates shear stress to velocity gradient.

Key knowledge

For simple shear in a Newtonian fluid, τ_xy=η∂v_x/∂y. A horizontal rigid circular tube in steady fully developed incompressible laminar flow with no slip has volume flow Q=πR⁴(p_in−p_out)/(8ηL). Tube radius strongly affects flow, while pressure drop supplies viscous dissipation.

Values and equations

τ_xy=η∂v_x/∂y; Q=πR⁴(p_in−p_out)/(8ηL)

τ_xy: x-directed viscous shear stress on a y-normal plane Pa
η: Dynamic viscosity Pa s
v_x: Fluid x velocity m s⁻¹
y,R,L: Transverse coordinate, tube radius and length m
Q: Volume flow rate, positive from inlet to outlet m³ s⁻¹
p_in,p_out: Inlet and outlet pressures Pa

Conditions and scope

η>0 constant; simple Cartesian shear for the constitutive equation. The tube has uniform R>0 and L>0, fixed walls, negligible entrance effects and no imposed body-force pressure contribution.
The tube law does not apply unchanged to turbulent flow, slip, non-Newtonian fluids or pulsatile/deforming vessels; laminar is not a universal fixed-Reynolds-number guarantee.

Sources · 1

References

Sources retain their recorded scope. A source link alone does not establish verification of every statement.

University Physics Volume 1, §14.7: viscosity and turbulenceSamuel J. Ling, Jeff Sanny and William Moebs — OpenStax, Rice University, University Physics (2016)Textbook chapteropenstax.orgRead source

Evidence and review

Read §14.7 parallel-plate shear law14.17 and Laminar Flow Confined to Tubes, Poiseuille resistance/flow14.22–14.23. The gradient form is the stated simple-shear extension; do not adopt universal Reynolds thresholds or imply that fluids have no viscous shear stress.

2026-10-10 · /root/audit_life_earth_chemistry — independent Chemistry/Life/Earth editor. Independent AI editorial review of this entry's statements, equations, applicability conditions and source coverage. Source checks apply to these claims within their stated limits.

Entry details

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Record identity: foundation-physics-viscosity-poiseuille
Revision: foundations-refresh.1
Structured reference record: Foundational reference additions