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1713, introduced as a term in physics 17c. by German astronomer and physician Johann Kepler (1571-1630), from Latin inertia "unskillfulness, idleness," from iners (genitive inertis) "unskilled, inactive;" see inert. Used in Modern Latin by Newton (1687). Sense of "apathy" first recorded 1822.
inertia in·er·tia (ĭ-nûr'shə)
The tendency of a body to resist acceleration; the tendency of a body at rest to remain at rest or of a body in motion to stay in motion in a straight line unless acted on by an outside force.
Resistance or disinclination to motion, action, or change.
The resistance of a body to changes in its momentum. Because of inertia, a body at rest remains at rest, and a body in motion continues moving in a straight line and at a constant speed, unless a force is applied to it. Mass can be considered a measure of a body's inertia. See more at Newton's laws of motion, See also mass.