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Inertia is a non-quantifiable property of matter (not a unit of measurement) by which it remains at rest
or in uniform motion in the same straight line unless acted upon by some external force. The principle of inertia is one of the fundamental principles of classical physics which are used to describe the motion of matter and how it is affected by applied forces. Today, it is most commonly defined using Sir Isaac Newton's third definition in Philosophiae Naturalis Principia Mathematica which states:
“ The vis insita, or innate force of matter is a power of resisting, by which every body as much as in it lies, continues in its present state, whether it be of rest, or of moving uniformly nor a measure of mass. ”
In common usage, however, people may also use the term "inertia" to refer to an object's "amount of resistance to change in velocity" (which is quantified by its mass), and sometimes its momentum, depending on context (e.g. "this object has a lot of inertia"). The term "inertia" is more properly understood as a shorthand for "the principle of inertia as described by Newton in Newton's First Law of Motion which, expressed simply, says: "An object that is not subject to any outside forces moves at a constant volocity, covering equal distances in equal times along a straight-line path." In even simpler terms, inertia means
"A body in motion tends to remain in motion, a body at rest tends to remain at rest." On the surface of the Earth the nature of inertia is often masked by the effects of friction which brings moving objects to rest relatively quickly unless they are coasting on wheels, well lubricated or perhaps falling or going downhill, being accelerated by gravity. This is what mislead classical theorists such as Aristotle who believed objects moved only so long as force was being applied to them
... whoop de doo, nothing will happen.
