Two-body reduction and reduced mass
For two Newtonian particles with equal opposite internal force and no external force, the centre of mass moves uniformly.
Key knowledge
For two Newtonian particles with equal opposite internal force and no external force, the centre of mass moves uniformly. Their relative vector r=r₂−r₁ satisfies μr̈=F₂ with reduced mass μ=m₁m₂/(m₁+m₂); this separates overall translation from internal motion.
Values and equations
μ=m₁m₂/(m₁+m₂); μr̈=F₂
- μ,m₁,m₂
- Reduced/particle masses kg
- r
- Relative separation vector m
- F₂
- Force on particle 2 N
Conditions and scope
- Constant positive masses and instantaneous Newtonian equal-opposite internal force.
- Central force is an additional assumption, not required for the reduction alone; field momentum or relativistic interaction requires a different treatment.
Sources · 1
References
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Evidence and review
Survey of Physics: Two-body dynamics: Equations 1.432–1.438: centre-of-mass and relative-coordinate reduction.
2026-10-10 · Independent AI editorial review: All 136 unique authored Physics compact statements, all hypotheses and displayed equations/variable/units tables, all 192 Physics-origin semantic links, exact 165 author inspection matrix and mapped retained conceptual claims have been independently read. Three Physics-tagged Measurement realization records are delegated to root Measurement review. Scientific source verification combines exact author section inspection with independent whole compact-content/semantic review and targeted primary rechecks recorded below; it does not claim each full source body was independently reread. Independent AI editorial review is not human specialist certification or experimental replication. Field inventory is approved after independently reviewing the two added source-inspected band-construction methods. Receipt: physics-independent-content-inventory-review.1.
Entry details
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