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Inertia is a body's persistence in its state of rest or constant velocity when the net external force vanishes in an inertial frame. It is not an extra force that pushes a moving body forward. Newton's first law describes this persistence; for a point mass, Newton's second law quantifies how a nonzero net external force changes its momentum. At constant mass, ${\mathbf{F}}_{\mathrm{net}}=m\mathbf{a}$, so a larger mass gives a smaller acceleration for the same net force.

A moving cart does not need a forward net force to maintain its velocity. Ordinary carts slow because friction and drag force produce an opposing net external force; reducing those interactions reveals Newton's first law more clearly. The air-track test of inertial motion separates this persistence from the initial push. In overdamped particle dynamics, the inertial acceleration term is small compared with drag force over the time scales considered; that approximation does not mean that inertia has disappeared.

A body exchanging mass with material of inertial velocity $\mathbf{u}$ satisfies the displayed momentum conservation law for one stream, with signed $\dot{m}$ positive for entry and negative for exit. The force $\mathbf{F}$ excludes the incoming or outgoing momentum flux. A short time interval gives $d(m\mathbf{v})/dt=\mathbf{F}+\mathbf{u}\dot{m}$, proving the formula. For stationary incoming matter, $d(m\mathbf{v})/dt=\mathbf{F}$; for matter leaving at the body's own velocity, $m\dot{\mathbf{v}}=\mathbf{F}$. The rocket equation and accreting-drop motion are applications.

Online text: farside.ph.utexas.edu/teaching/jk1/Electromagnetism/node53.html

Cited passages: Localized Current Distribution, equations 661–677: distant vector-potential expansion and dipole field; Localized Current Distribution, equations 678–679: closed-loop integral and planar area normal; Richard Fitzpatrick, Localized Current Distribution, equations 664–675: vanishing boundary terms, current identities and moment integral.

Online text: farside.ph.utexas.edu/teaching/em/lectures/node74.html

Cited passages: Richard Fitzpatrick, Magnetic susceptibility and permeability, equations (866)–(868): isotropic linear response and permeability.

Online text: farside.ph.utexas.edu/teaching/em/lectures/node69.html

Cited passages: Richard Fitzpatrick, Polarization, equations (807)–(812): displacement, polarization and susceptibility; this source uses epsilon for relative permittivity.

A chromatid is one of the two copies of a replicated chromosome. Each homologous chromosome contributes two sister chromatids before separation in meiosis; chromosomal crossovers can exchange segments between nonsister chromatids.

// Target: biology.bigb

Homologous chromosomes carry corresponding genetic loci, although their alleles may differ. In the diploid $AB/ab$ example, the two homologous chromosomes carry the parental haplotypes $AB$ and $ab$.

A thylakoid is a membrane-enclosed sac; its thylakoid membrane contains chlorophyll and light-dependent reactions machinery. A stack is a granum.^[ref]

The membrane and its enclosed fluid answer different location questions. A soluble protein in thylakoid lumen is inside the sac; a membrane protein is embedded in its boundary and can expose different parts on the two sides. Membrane topology and organelle protein import provide the reusable language for those distinctions.