= Friction
<Friction> is a contact <force> tangent to the surfaces. <Kinetic friction> opposes <contact slip>; <static friction> maintains contact without <contact slip>, including <rolling without slipping>. The perpendicular contact component is the <normal force>. These two components answer different questions: how strongly the surfaces press together, and how strongly they resist sliding past one another.
The ideal Coulomb dry-contact model combines a bound on <static friction> with a sliding law for <kinetic friction>. Its parameters are the <coefficient of static friction> and the <coefficient of kinetic friction>; constant coefficients are an empirical approximation, rather than a consequence of <Newton's laws of motion>. Surface condition matters, and the static coefficient need not universally exceed the kinetic coefficient. https://www.feynmanlectures.caltech.edu/I_12.html#Ch12-S2[Feynman, Leighton and Sands, The Feynman Lectures on Physics I, §12-2: friction and the limits of the empirical law]{ref}
The model's historical setting was practical machinery: Coulomb's \i[Théorie des machines simples] investigated <friction> and the stiffness of ropes. The Académie's bibliography records its 1781 prize and its appearance in volume 10, pages 161–332, of the \i[Recueil des savants étrangers]. https://www.academie-sciences.fr/pdf/dossiers/Coulomb/Coulomb_pubd1.htm[Académie des sciences: Coulomb's published works]{ref}
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