Inertia is a non-quantifiable property of matter 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 first in Philosophiae Naturalis Principia Mathematica which states: 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

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