= Classical mechanics
{wiki}
<Classical mechanics> describes the motion of bodies by assigning <positions> and <velocities> and specifying how their interactions change those quantities. A <point mass> is the simplest model: its size and internal structure are ignored, leaving a <mass> and a moving <position>. A <rigid body> retains orientation and <rotational kinetic energy>. Choosing a model means deciding which features affect the question being asked; a sliding block may be treated as a <point mass>, whereas predicting its <rolling without slipping> requires its <moment of inertia>.
Three complementary questions organize the subject. <Newton's second law> asks how the net <force> determines the instantaneous <acceleration>. The <work-energy theorem> asks how that <force>, acting through a <displacement>, changes <kinetic energy>. <Conservation of mechanical energy> asks which combinations of <position> and <velocity> are accessible when the interactions admit a time-independent <potential energy> and no other <force> supplies net <power>. These descriptions are compatible, but answer different questions: an <energy> calculation may find a maximum <speed> without first solving for <position> at every <time>.
The <harmonic oscillator> below makes this unity visible. A restoring <force> produces a repeating <position> trace; the corresponding <phase plane> has a closed <ellipse>; <kinetic energy> and <potential energy> exchange while their sum stays fixed. <Lagrangian mechanics> and <Hamiltonian mechanics> provide further ways to organize the same dynamics, especially when coordinates or constraints make individual <forces> awkward to track.
The scope of <classical mechanics> also matters. <Newtonian mechanics> approximates motion at <speeds> small compared with the <speed of light>; <special relativity> changes that description when this approximation fails. <Quantum mechanics> is needed for phenomena for which a classical assignment of simultaneous <position> and <momentum> is inadequate. These limitations do not prevent <classical mechanics> from being a useful predictive model within its regime. https://ocw.mit.edu/courses/8-01sc-classical-mechanics-fall-2016/mit8_01scs22_chapter1.pdf[MIT mechanics, Chapter 1: scope and limitations]{ref}
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