Inertia (source code)
= Inertia
<Inertia> is a body's persistence in its state of rest or constant <velocity> when the net <external force> vanishes in an <inertial frame>. It is not an extra <force> that pushes a moving body forward. <Newton's first law> describes this persistence; for a <point mass>, <Newton's second law> quantifies how a nonzero net <external force> changes its <momentum>. At constant <mass>, $\mathbf F_{\mathrm{net}}=m\mathbf a$, so a larger <mass> gives a smaller <acceleration> for the same net <force>.
A moving cart does not need a forward net <force> to maintain its <velocity>. Ordinary carts slow because <friction> and <drag force> produce an opposing net <external force>; reducing those interactions reveals <Newton's first law> more clearly. The <air-track test of inertial motion> separates this persistence from the initial push. In <overdamped particle dynamics>, the inertial <acceleration> term is small compared with <drag force> over the <time> scales considered; that approximation does not mean that <inertia> has disappeared.
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