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momentum

/moʊˈmentəm//məˈmentəm/·noun
the force that keeps something moving or developing once it has started
Fig. 1 — I watch a heavy, loaded freight wagon already rolling along a level track, with nothing pushing it.
01Definition

Momentum is motion's refusal to stop. In physics it is mass times velocity — the measure of how hard a moving thing is to halt — and the figurative sense keeps that arithmetic: a campaign, reform, or career 'gains momentum' as each success makes the next one cheaper, until the effort is no longer in moving but in stopping. From the Latin movere, 'to move'. Momentum is borrowed force: the push you gave yesterday, still paying for today's progress.

02In use
  • iThe campaign gained momentum after the first televised debate.
  • iiReforms tend to surge while a crisis lasts, then lose momentum once it passes.
  • iiiThe boulder rolled downhill, gathering momentum until nothing could impede it.
03Collocations
  • gain momentum
  • gather momentum
  • lose momentum
  • maintain the momentum
  • the momentum behind
04Relations

=impetus, drive, thrust, traction, propulsion

≠standstill, stagnation, inertia

05Compared
PairThe difference
momentum vs impetusthe push that starts the motion → impetus; the force that keeps it going → momentum.
06TOEFL & IELTS

Double-duty vocabulary: TOEFL physics lectures use the literal sense, while history and social passages narrate movements that 'gained momentum' — a signpost that change is accelerating. For IELTS essays, 'maintain the momentum of reform' is a high-band phrase. The testable part is the verb company it keeps: gain, gather, build, lose, sustain. No useful derived forms — 'momentous' means important, a different word in practice.

07Asked
What does it mean to gain momentum?
To reach the point where progress increasingly carries itself: early on, every advance needs a fresh push; past it, the thing moves largely because it is already moving. A campaign gains momentum when volunteers begin recruiting volunteers. The scene above shows the end state — a loaded wagon rolling on level track with nothing pushing it, the mass having taken over the job the push once did.
Is momentum the same as impetus?
They divide the journey between them: impetus is the shove that starts a thing, momentum is what keeps it rolling once started — the pair is set side by side on the impetus vs momentum page in the 'Compared' list above.
What is the difference between momentum and inertia?
In everyday English they point opposite ways. Inertia is resistance to change and almost always means staying put: bureaucratic inertia is why nothing moves. Momentum is motion already underway and the difficulty of stopping it. Physics defines both more evenhandedly — inertia resists any change of state, moving or still — but as vocabulary, inertia parks the car and momentum refuses to brake.
Is the plural momenta or momentums?
Physics keeps the Latin plural: the momenta of two colliding bodies. Outside physics the question rarely arises, because figurative momentum behaves as an uncountable noun — a movement gains momentum, never 'momentums'. If you are ever forced to count, momentums is recorded and understood, but recasting the sentence is almost always the better repair.
Are momentum and momentous related?
At the root, yes: Latin momentum — apparently a worn-down form of an older movimentum, from movere, 'to move' — meant both movement and the importance that tips a scale, and English split the inheritance. Momentum took the motion; momentous took the weight: a momentous decision is an important one, not a fast-moving one. The overlap survives only in the image of scales tipping.