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h3. Definition and Law of Change     [!images^SectionEdit.png!|Momentum (Intro)]

Although Newton is famous for the law {*}F = ma{*}, he actually stated his Second Law in terms of momentum and its change due to impressed forces or impulses (the time integral of the force).   He defined {*}_Momentum_{*} as
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_The [quantity of motion] is the measure of the same, arising from the [velocity] and quantity of matter conjointly_. - {*}DEFINITION II, Principia{*} (Motte and Cajori).  

In modern terminology, this definion becomes:

{latex}\begin{large}\[ \vec{p} = m\vec{v} \]\end{large}{latex}


[Newton's Second Law] can be written in terms of momentum, and represents the fundamental law of change for the [Momentum and External Force] [model]:

{latex}\begin{large}\[ \sum_{\rm ext} \vec{F} = \frac{d\vec{p}}{dt}\]\end{large}{latex}

This form is totally equivalent to {*}_F_ = _ma_{*}, provided that the [system] under consideration has constant [mass].  Unlike the traditional {*}_F_ = _ma_{*} formulation, however, the formulation of Newton's Second Law in terms of momentum is accurate even for systems whose mass is variable.