Momentum (physics)
Etymology
From Latin momentum, "movement, moving power," contracted from movimentum (movere, "to move"). The same root gave us "moment," and the two senses — a decisive instant and a driving force — both live in the word. Physics fixed the technical meaning: the conserved measure of a body’s motion.
noun · /moh-MEN-tum/
The quantity of motion a moving thing carries — its mass times its velocity — and thus how hard it is to stop or turn.
What it names
Why efforts that are already moving tend to keep moving, so getting started is the hard part and staying started is comparatively easy.
Example
After three days at the gym in a row she had a little momentum, and skipping the fourth felt harder than going.
Related idea
Building momentum means accumulating motion that carries you forward; overcoming inertia means supplying the first push to a thing that is not moving at all.
noun · /moh-MEN-tum/
The driving force or impetus a process gathers as it goes, making it harder to stop or reverse.
What it names
figurative unstoppability — the growing push behind a campaign, movement, or trend
Example
After three wins the team carried real momentum into the finals.
Related idea
The physics quantity becomes metaphor: a movement, like a mass in motion, resists being halted once it is rolling — close to inertia, though inertia resists starting as much as stopping.