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Showing 1–30 of 365 reference entries

Mathematics & logic · Physics · MeasurementRatios and rates

Distinguish part-to-part ratios, fractions of a whole and rates with units. Scale quantities together, compare unit rates, and explain when a relationship is proportional. Check zero cases, starting amounts and whether a result fits its context.

DefinitionMathematics & logic · Physics · MeasurementQuantities and units

Describe a quantity as a numerical value with a unit, and distinguish its physical kind from its dimension. Build length, area, volume and rate units, check meaningful combinations, and separate exact unit relations from measurement uncertainty.

DefinitionMathematics & logic · Physics · MeasurementUnit conversions

Convert quantities with exact unit ratios, track cancellation through compound units, and apply the correct powers to area and volume. Distinguish temperature readings from differences, check dimensions and context, and retain measurement uncertainty when reporting.

MethodMathematics & logic · Physics · MeasurementEstimation and uncertainty

Distinguish exact model values, rounded approximations and estimates from measurements. Declare the rounding rule, preserve information through calculations and unit changes, and interpret resolution, precision, accuracy and the stated uncertainty type without turning a report’s digits into guaranteed bounds.

DefinitionMathematics & logic · PhysicsCoordinates and tables

Locate a point from an ordered pair, read named axes and their scales, and connect the same values in words, tables and plots. Keep signs, fractions and units exact, distinguish an axis point from a quadrant point, and separate selected observations from a stated rule and its domain before drawing connections.

DefinitionMathematics & logic · PhysicsSlope and straight lines

Read a straight line as a stated relationship, calculate its slope from consistent signed coordinate changes, and interpret its intersections with the axes. Keep horizontal, vertical and coincident-point cases distinct, attach units and domain conditions to context models, and use numerical scales rather than appearance alone to judge a line’s change.

DefinitionMathematics & logic · Physics · MeasurementReading graphs

Read a graph’s quantities, units, ticks and context before judging its visual impression. Calculate differences, reference-based percentages and interval rates from the represented values; distinguish legitimate zoom from distorted spacing; and keep samples, interpolation, missing information, uncertainty and causal explanations separate.

MethodPhysics · MeasurementObservables and measurement

Choose the physical quantity you want to investigate, specify the point, event or location involved, and explain how a reading relates to that quantity.

MethodPhysicsSystems and boundaries

Choose what belongs to an investigation, describe its condition at a stated time, and identify interactions with what lies outside its boundary.

MethodPhysicsModels and assumptions

Choose what a model represents, make its simplifications explicit, and check whether its predictions answer the physical question you actually asked.

MethodPhysicsHypotheses, laws and theories

Identify what a scientific statement does, examine its evidence and conditions, and avoid treating its name as a score for certainty.

MethodMathematics & logic · PhysicsApproximations

Build a useful estimate, compare a correction with a stated reference scale, and check what is lost when you leave out a term.

ApproximationMathematics & logic · PhysicsSymmetry and invariance

Specify a transformation, identify what stays unchanged, and distinguish symmetry of an object from invariance of a physical model.

Mathematical statementPhysicsConservation laws

Use a defined system and a named quantity to track transfers, distinguish changing parts from conserved totals, and check the conditions of energy and momentum balances.

ModelPhysicsEmergence and levels of description

Choose a descriptive level, connect collective quantities with component behavior, and check when a simpler physical description answers a specified question.

ModelMathematics & logic · PhysicsVectors

Describe displacement with fixed-axis components, combine compatible vectors, and use small exact examples to introduce span, independence, basis and real vector spaces.

DefinitionPhysics · Information · MeasurementScientific records

Build an inspectable physics record, test and revise a bounded cart model, preserve processing history, and plan findable data with appropriate sharing conditions.

MethodPhysicsPosition, velocity and acceleration

Describe a point's motion on a chosen straight axis, then connect signed changes, instantaneous rates and one exact set of position, velocity and acceleration graphs.

DefinitionPhysicsNewton’s laws and forces

Choose one body, identify the forces acting on it, and use their vector sum to distinguish acceleration, balanced motion and mutual interactions.

ModelPhysicsWork, energy and power

Connect force and displacement to changes in motion, follow a spring’s stored energy, and distinguish an energy transfer from its rate.

ModelPhysics · MeasurementSpeed of light in vacuum

c denotes the speed of light in vacuum. Its numerical value is fixed exactly in SI units.

DefinitionPhysics · MeasurementPlanck constant

h denotes the Planck constant. Its numerical value is fixed exactly in SI units.

DefinitionPhysics · MeasurementCaesium-133 defining frequency

ΔνCs denotes the unperturbed ground-state hyperfine transition frequency of caesium-133. Its numerical value is fixed exactly in SI units.

DefinitionMathematics & logic · PhysicsSine, cosine and the unit circle

Sine and cosine are unit-circle coordinates whose squared values sum to one.

Mathematical statementMathematics & logic · Information · PhysicsLinear maps and matrix representation

A linear map preserves linear combinations and has a matrix once bases are chosen.

DefinitionMathematics & logic · PhysicsDerivative at a point

A derivative is a finite limit of local difference quotients.

DefinitionMathematics & logic · PhysicsDefinite integral and signed accumulation

A definite integral is the limiting signed accumulation of small contributions.

DefinitionMathematics & logic · PhysicsFundamental theorem of calculus

Continuity connects differentiation with integration.

Mathematical statementMeasurement · Physics · Mathematics & logicDimensional consistency

Use dimensions to detect impossible quantity equations.

MethodPhysics · Earth & cosmosNewtonian gravitation

An inverse-square force models attraction between point masses.

Model
MethodSource checked

Systems and boundaries

Physics

Choose what belongs to an investigation, describe its condition at a stated time, and identify interactions with what lies outside its boundary.

Key knowledge

Describing a system requires identifying its components and interactions, its boundary, and relevant inputs and outputs. What counts as internal or external depends on that chosen boundary.

Conditions and scope

  • A boundary is part of the model and must be stated before applying a balance law.
Sources · 13

References

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

  1. Massachusetts Institute of Technology2 sources
    1. Lectures on Thermodynamics, Chapter 1: Energy Accounting, Variables and Properties of SystemsGerbrand Ceder · Massachusetts Institute of TechnologyCourse notes · PDFocw.mit.eduRead source
    2. MIT Unified Engineering Thermodynamics — State, heat and workIan A. Waitz (course instructor) · Massachusetts Institute of TechnologyCourse notesUnified Engineering Thermodynamics, Chapter 2: Thermodynamics ConceptsUnified Engineering Thermodynamics, Chapter 3: Changing the State of a System with Heat and Workocw.mit.edu
  2. NPTEL Advanced Engineering Thermodynamics — Systems and exchange classificationP. Mahanta · Indian Institute of Technology Guwahati / NPTELCourse notesAdvanced Engineering Thermodynamics, Module 1: Classical Thermodynamics, Lecture 1: Review of Thermodynamics — Concepts of classical thermodynamicsAdvanced Engineering Thermodynamics, Module 1: Classical Thermodynamics, Lecture 1: Review of Thermodynamics — System approach and classification of systemarchive.nptel.ac.in
  3. University Physics Volume 1, 3.1 Position, Displacement, and Average VelocityWilliam Moebs, Samuel J. Ling, Jeff Sanny · OpenStax / Rice UniversityTextbook chapteropenstax.orgRead source
  4. Introduction to Engineering Thermodynamics — Systems, state and boundary heatClaire Yu Yan · University of British Columbia Okanagan / BCcampusTextbook chapterIntroduction to Engineering Thermodynamics, 1.2 System and surroundingsIntroduction to Engineering Thermodynamics, 1.4 State, process, and cycleIntroduction to Engineering Thermodynamics, 4.2 Heat transfer across a boundarypressbooks.bccampus.ca
  5. NGSS Appendix G: Crosscutting Concepts (2013)NGSS Lead States — Next Generation Science StandardsTeaching resource · PDFnextgenscience.orgRead source
  6. University Physics, volume 1: Newton's Second LawSamuel J. Ling, Jeff Sanny and William Moebs / OpenStax, Rice UniversityTextbook chapteropenstax.orgRead source
  7. Systems and conservation of momentum · Physical Sciences 12SiyavulaTextbook chaptersiyavula.comRead source
  8. Basic Thermodynamics · Module1K. Srinivasan and Pradip Dutta · IIScCourse notes · PDFarchive.nptel.ac.inRead source

Evidence and review

NGSS Appendix G, systems and system models; OpenStax 5.3, internal and external forces.

2026-10-10 · System-boundary convention only; no claim that a selected boundary makes a system isolated.

Foundational review

2026-10-10 · Fresh primary-source inspection of the complete compact record, including every statement, equation, quantity and material condition, followed by independent AI editorial review. This check does not certify all linked lessons or every passage of every cited publication.

Sources from other places

Agreement within this scope

Choosing a system boundary determines which components, interactions and transfers count as internal or external.

A boundary is chosen for the problem; matter and energy exchanges then belong in the system description.

Related reference entries

Explore relationships within this area of study and its connections to other areas.

Showing 1–13 of 13 relationships

Scientific relationships · 8 / 8

The relationship type describes the connection. Its linked statements, sources and conditions define the scope.

  1. Uses definition
    Relationship sources · 1

    Heat and work cross the specified closed-system boundary under the adopted sign convention.

  2. Uses definition
    Relationship sources · 2

    The total entropy inequality applies to the whole isolated system, not each arbitrary subsystem.

  3. Uses definition
    Relationship sources · 2

    Linked statements

    • A free-body diagram represents all external forces acting on the selected body or system. Forces the system exerts on its surroundings belong on another system’s diagram. A force and its components are alternative representations, and the net force is their sum rather than an extra force. For an extended body, force locations are also needed to determine torque.

    This relation concerns the stated free-body diagrams and system boundaries claim and its conditions; it does not assert an unqualified prerequisite or an additional theorem.

  4. Uses definition
    Relationship sources · 1

    Linked statements

    • For a Newtonian particle under time-independent conservative forces plus other forces, E_mech=K+U and ΔE_mech=W_other. When the other forces do zero work, mechanical energy stays constant. Friction can convert ordered mechanical energy into internal energy; loss of K+U is compatible with total-energy conservation for a correctly chosen combined system.

    This relation concerns the stated mechanical-energy conservation and dissipation claim and its conditions; it does not assert an unqualified prerequisite or an additional theorem.

  5. Uses definition
    Relationship sources · 2

    Linked statements

    • For a mechanically quasistatic simple compressible closed system, work done by the system is W=∫p dV: expansion contributes positive work and compression negative work. A quasistatic isothermal ideal gas with fixed amount gives W=nRTln(V_f/V_i). Isochoric means constant volume; isobaric constant pressure; isothermal constant temperature; adiabatic means no heat transfer.

    This relation concerns the stated pressure–volume work and process paths claim and its conditions; it does not assert an unqualified prerequisite or an additional theorem.

  6. Uses definition
    Relationship sources · 2

    Linked statements

    • A reversible ideal process can be reversed so both system and surroundings return to their original states. A quasistatic model moves through a succession of nearly equilibrium states, allowing state variables along the path; slowness alone does not remove friction or finite-gradient entropy production. Free expansion of an insulated ideal gas can leave its temperature unchanged while remaining irreversible.

    This relation concerns the stated reversible and quasistatic processes claim and its conditions; it does not assert an unqualified prerequisite or an additional theorem.

  7. Uses definition
    Relationship sources · 2

    Linked statements

    • A measuring instrument is a device used to make measurements, alone or with supplementary devices. A measuring system is a set of one or more instruments and often other devices, reagents or supplies assembled to give quantity values within specified intervals for specified kinds of quantity. A single instrument can itself be a measuring system when it fulfils that role.

    This relation concerns the stated measuring instrument and measuring system claim and its conditions; it does not assert an unqualified prerequisite or an additional theorem.

  8. Uses definition
    Relationship sources · 3

    Linked statements

    • An open thermodynamic system permits matter to cross its boundary; a closed system permits no matter transfer but may exchange energy. An isolated system exchanges neither matter nor energy. Adiabatic means no heat transfer during the process: work can still transfer energy, so an adiabatic closed system need not be isolated.

    A thermodynamic boundary is a particular application of the existing system-boundary definition.

Learning guidance · 5 / 5

Preserved learning suggestions. They are not proof, empirical evidence or mandatory prerequisites.

  1. Helpful before · learning guidance
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Physical systems, states, boundaries, and environments ↗
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systems
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systems