Force

=== First law ===
{{main|Newton's first law}}
Newton's first law of motion states that the natural behavior of an object at rest is to continue being at rest, and the natural behavior of an object moving at constant speed in a straight line is to continue moving at that constant speed along that straight line.<ref name="Principia"/> The latter follows from the former because of the [[principle of relativity|principle that the laws of physics are the same]] for all [[inertial frame of reference|inertial observers]], i.e., all observers who do not feel themselves to be in motion. An observer moving in tandem with an object will see it as being at rest. So, its natural behavior will be to remain at rest with respect to that observer, which means that an observer who sees it moving at constant speed in a straight line will see it continuing to do so.<ref name="mermin2005">{{cite book|first=N. David |last=Mermin |author-link=N. David Mermin |title=It's About Time: Understanding Einstein's Relativity |publisher=Princeton University Press |year=2005 |isbn=978-0-691-21877-9}}</ref>{{rp|1–7}}

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[[File:GodfreyKneller-IsaacNewton-1689.jpg|upright|thumb|[[Sir Isaac Newton]] in 1689. His ''Principia'' presented his three laws of motion in geometrical language, whereas modern physics uses [[differential calculus]] and [[Vector (mathematics and physics)|vector]]s.]]