Conservation 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.
Key knowledge
For a fixed collection of classical particles whose internal forces satisfy Newton's third law, zero net external force keeps the total linear momentum constant.
Values and equations
P = Σᵢ mᵢvᵢ; F_external,net = 0 ⇒ dP/dt = 0
- P
- Total vector linear momentum kg m s⁻¹
- mᵢ
- Mass of particle i kg
- vᵢ
- Vector velocity of particle i m s⁻¹
- F_external,net
- Vector sum of external forces N
- t
- Time s
Conditions and scope
- Use an inertial frame and a fixed collection of particles.
- Internal forces obey equal-and-opposite pairing; all external forces are included in the total.
Sources · 7
References
Sources retain their recorded scope. A source link alone does not establish verification of every statement.
- The International System of Units (SI), 9th edition, version 4.01Bureau International des Poids et Mesures (BIPM)Reference guide
- Classical Thermodynamics, Module 1, Lecture 1: Review of Thermodynamics — System approach and classification of systemNPTELCourse notes
- University Physics Volume 2, 3.3 First Law of ThermodynamicsSamuel J. Ling, William Moebs and Jeff Sanny — OpenStax / Rice UniversityTextbook chapter
- Physics Part I, Class XI, Chapter 5: Work, Energy and PowerNational Council of Educational Research and Training (NCERT), IndiaTextbook chapter · PDF
- University Physics Volume 1, 9.3 Conservation of Linear MomentumWilliam Moebs, Samuel J. Ling and Jeff Sanny — OpenStax / Rice UniversityTextbook chapter
- University Physics Volume 3, 5.9 Relativistic EnergySamuel J. Ling, Jeff Sanny and William Moebs — OpenStax / Rice UniversityTextbook chapter
- Systems and conservation of momentum · Physical Sciences 12SiyavulaTextbook chapter
Evidence and review
OpenStax 9.3: summed forces, fixed system and zero net external force.
2026-10-10 · Classical linear-momentum conservation under stated conditions only; relativistic field momentum and other conserved quantities are outside scope.
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
Classical total vector momentum remains constant when external resultant force vanishes and internal force pairs cancel.
The isolation condition is essential; momentum of a colliding subsystem can change through external forces.
Siyavula
- Systems and conservation of momentum · Physical Sciences 12 South AfricaEnglish
Conservation of momentum, ESCJF; isolated system and force cancellation
- Systems and conservation of momentum · Physical Sciences 12
Related articles
Conservation laws ↗Entry details
Permanent link to this entry ↗- Record identity
- conservation
- Revision
- reference-core.1
- Published lesson seed
- conservation