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'''Inertia''' is the [[resistance]] of any [[physical]] object to a [[change]] in its state of [[relative]] [[motion]]. It is represented numerically by an object's [[mass]]. 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]]. Inertia comes from the [[Latin]] word, "iners", meaning idle, or lazy. Sir [[Isaac Newton]] defined inertia in Definition 3 of his Philosophiæ Naturalis Principia Mathematica, which states:[1]

'''Inertia''' is the [[resistance]] of any [[physical]] object to a [[change]] in its state of [[relative]] [[motion]]. It is represented numerically by an object's [[mass]]. 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]]. Inertia comes from the [[Latin]] word, "iners", meaning idle, or lazy. Sir [http://en.wikipedia.org/wiki/ Isaac Newton] defined inertia in Definition 3 of his Philosophiæ Naturalis Principia Mathematica, which states:[1]

*The vis insita, or innate [[force]] of [[matter]] is a [[power]] of resisting, by which every [[body]], as much as in it lies, endeavors to preserve in its [[Status quo|present state]], whether it be of rest, or of moving [[uniformly]] forward in a straight line.

:The vis insita, or innate [[force]] of [[matter]] is a [[power]] of resisting, by which every [[body]], as much as in it lies, endeavors to preserve in its [[Status quo|present state]], whether it be of rest, or of moving [[uniformly]] forward in a straight line.

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), or sometimes to its [[momentum]], depending on the [[context]] (e.g. "this object has a lot of inertia"). The term "inertia" is more properly understood as shorthand for "the principle of inertia" as described by Newton in his ''[http://en.wikipedia.org/wiki/Newton%27s_laws_of_motion First Law of Motion]''. This [[law]], [[expressed]] simply, says that an object that is not subject to any net external force moves at a constant velocity. In even simpler terms, inertia means that an object will always continue moving at its current speed and in its current direction until some force causes its speed or direction to [[change]]. This would include an object that is not in motion (velocity = zero), which will remain at rest until some force causes it to move.

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), or sometimes to its [[momentum]], depending on the [[context]] (e.g. "this object has a lot of inertia"). The term "inertia" is more properly understood as shorthand for "the principle of inertia" as described by Newton in his ''[http://en.wikipedia.org/wiki/Newton%27s_laws_of_motion First Law of Motion]''. This [[law]], [[expressed]] simply, says that an object that is not subject to any net external force moves at a constant velocity. In even simpler terms, inertia means that an object will always continue moving at its current speed and in its current direction until some force causes its speed or direction to [[change]]. This would include an object that is not in motion (velocity = zero), which will remain at rest until some force causes it to move.