ISO 80000-4: Quantities and Units of Mechanics

ISO 80000-4 codifies the vocabulary of mechanics so mass properties, forces, and energy flows speak the same language across industries. Use this guide when documenting structural loads, rotating equipment, or dynamic simulations that rely on coherent SI units.

Start with the foundational rules in ISO 80000-1 and the typography conventions from ISO 80000-2 so every symbol and index in your reports is compliant. When motion enters the picture, revisit ISO 80000-3 for velocity and acceleration definitions that feed the force equations below.

Need a panoramic view first? Explore the ISO 80000 overview and the part-by-part quick tables before drilling into the mechanical tables below. As you apply the terminology, launch calculators like the force from mass and acceleration tool or the torque converter to keep unit conversions transparent. When mass and volume reporting intersect, consult the density explainer so bulk material specifications reference ISO 80000-12 conventions alongside the mechanical framework.

Dive deeper into Part 4’s most referenced derived units with our newton, joule, and pascal explainers whenever you need narrative context, historical milestones, or calculator shortcuts that show these definitions in action.

Working on dynamic similarity or compressible-flow problems that pull on mechanical quantities? Pair this article with the Reynolds number and Mach number deep dives to keep inertia, pressure, and speed terminology aligned with ISO 80000-4 conventions.

How ISO 80000-4 structures mechanics

The standard organises mechanical science around the relationships between mass, force, energy, and distributed loads. These clusters summarise the narrative used throughout the part.

  • Mass and inertia

    Explains how rest mass, moments of inertia, and density describe resistance to linear or angular acceleration in any mechanical system.

  • Dynamics and forces

    Connects Newton's second law to interaction forces, torques, and momentum balances that govern motion, machinery, and structural loads.

  • Energy transfer

    Defines work, kinetic and potential energy, and power so conversions between mechanical, electrical, and thermal domains stay coherent.

  • Continuum mechanics

    Covers pressure, stress, strain energy density, and impedance to ensure fluids and solids use compatible units in simulations and compliance reporting.

Principal ISO 80000-4 quantities and unit definitions

Each quantity below includes the coherent SI unit mandated by ISO 80000-4 together with plain-language definitions and example applications. Use the list as a checklist when drafting specifications, training materials, or data schemas for mechanics projects.

Quantity Symbol Coherent unit ISO 80000-4 definition Example application
Mass m kilogram (kg) Base quantity that measures inertia; ISO 80000-4 references the kilogram defined via the Planck constant so mechanical calculations remain traceable. Sizing counterweights or calculating gravitational force in conveyor design.
Linear momentum p newton second (N·s = kg·m/s) Product of mass and velocity describing translational motion; conservation rules underpin impact analysis and propulsion sizing. Determining braking distances for vehicles by comparing incoming and allowable momentum.
Angular momentum L joule second (J·s) Moment of momentum, combining moment of inertia and angular velocity; aligns gyroscopic behaviour with the SI action unit. Stabilising satellites or drones that rely on reaction wheels and control moment gyros.
Moment of inertia I kilogram metre squared (kg·m²) Integral of mass distribution relative to an axis; describes how difficult it is to change rotational speed. Selecting flywheels or robotic joints that must accelerate without overshooting torque limits.
Force F newton (N = kg·m/s²) Time rate of change of momentum; anchors load calculations, structural analysis, and machine safety checks. Computing actuator requirements or evaluating the load transmitted through a drive shaft.
Weight force W newton (N) Specific force resulting from gravitational acceleration acting on mass; ISO 80000-4 keeps it distinct from mass to prevent confusion in calculations. Translating mass inventory into supported loads for rigging, cranes, and storage racking.
Pressure / stress p, σ pascal (Pa = N/m²) Force distributed per unit area; applies both to fluid pressure and internal stresses in materials. Assessing hydrostatic pressure on tanks or verifying allowable stress in finite element models.
Work / energy W, E joule (J = N·m) Line integral of force along displacement or torque across angle; stores or transfers mechanical energy coherently with electrical and thermal joules. Calculating the energy budget of hoists, elevators, or regenerative braking systems.
Power P watt (W = J/s) Rate at which work is done or energy is transferred; essential for specifying motors, turbines, and compressors. Comparing motor nameplates or verifying efficiency ratings that must quote watts or kilowatts.
Torque (moment of force) M, τ newton metre (N·m) Vector product of position and force; determines twisting effect applied to shafts, fasteners, or joints. Setting tightening specifications for bolted connections or evaluating drivetrain loads.
Mechanical impedance Z newton second per metre (N·s/m) Ratio of force to velocity in linear systems; supports vibration isolation and acoustic-mechanical coupling analysis. Selecting damping elements to control resonance in precision manufacturing equipment.
Mass density ρ kilogram per cubic metre (kg/m³) Mass per unit volume, crucial for converting distributed loads into total forces within continuum mechanics. Estimating payload mass from tank volume in chemical process or aerospace applications.
Specific energy e joule per kilogram (J/kg) Energy content per unit mass, linking mechanics to thermodynamics when evaluating propulsion or storage systems. Comparing battery packs, flywheels, or compressed air reservoirs for mobile equipment.

Why these definitions matter

Applying ISO 80000-4 keeps design reviews, regulatory filings, and digital twins consistent across the mechanics lifecycle. These takeaways highlight the most consequential benefits for multidisciplinary teams.

Coherent newton–joule–watt ladder

ISO 80000-4 builds every work and power equation from the newton, ensuring mechanical energy integrates seamlessly with electrical or thermal energy balances documented in other ISO 80000 parts.

Momentum as a compliance anchor

By standardising linear and angular momentum units, the part guarantees that brake testing, crash simulations, and aerospace manoeuvres compare data without hidden conversion factors.

Stress language for materials and fluids

Shared pascal-based definitions mean structural engineers, fluid dynamicists, and safety regulators read the same numbers when reviewing load cases, fatigue curves, or hydraulic schematics.

Implementation checklist

Turning the standard into working practice means translating unit definitions into procedures, instrumentation, and analytics pipelines. Use these steps to keep teams aligned.

  1. Inventory load-bearing data

    List every spreadsheet, drawing, or SCADA tag that records mass, torque, or pressure. Flag values that still use legacy units such as pounds-force or bar so they can be normalised to SI form.

  2. Update instrumentation references

    Configure sensors, PLCs, and historian databases to log base SI units. Where displays require customary units, store the SI value and document the conversion factor alongside the tag description.

  3. Embed mechanical validation

    Integrate calculators and dimension checks into maintenance and design workflows. Require teams to cite ISO 80000-4 quantities when approving torque procedures, energy budgets, or pressure relief settings.

Reinforce adoption with training that references the International System of Units and practical exercises using our calculators so specialists and newcomers handle the same conversion factors.

Recommended follow-up reading

Expand beyond mechanical units with these in-depth explainers from the Units & Measures library.

Practice with calculators

Apply ISO 80000-4 immediately by checking your assumptions with these interactive tools from across CalcSimpler.

For broader digital transformation insights, continue exploring our newsroom or connect mechanics with finance workflows via the inventory turnover ratio calculator.