Related to the tendency of objects to stay at rest or keep moving unless a force acts on them; also means sluggish or lacking energy.
From Latin 'inertia' (sluggishness, idleness) + '-al' (adjective suffix). Newton formalized this as a physics law in the 1600s, giving the medieval Latin concept scientific precision.
Inertia explains both why you lurch forward when a car brakes suddenly AND why it's hard to get motivated on a Monday morning—the same physical principle applies to matter and to human motivation, which is why the word works for both.
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