Editing Momentum (section)

=== Quantity of motion<span class="anchor" id="Quantity of motion"></span> ===

==== René Descartes ====
In ''[[Principles of Philosophy]]'' (''Principia Philosophiae'') from 1644, the French philosopher [[René Descartes]] defined "quantity of motion" (''[[Latin language|Latin]]: quantitas motus'') as the product of size and speed,<ref name=":0">{{Cite book |last=Descartes |first=R. |url=https://www.earlymoderntexts.com/assets/pdfs/descartes1644part2.pdf |title=Principles of philosophy |year=2008 |editor-last=Bennett |editor-first=J. |at=Part II, § 36. |orig-date=1644}}</ref> and claimed that the total quantity of motion in the universe is conserved.<ref name=":0" /><ref>Alexander Afriat (2004). [http://philsci-archive.pitt.edu/1699/1/Momentum3.pdf "Cartesian and Lagrangian Momentum"]. {{webarchive|url=https://web.archive.org/web/20170309014638/http://philsci-archive.pitt.edu/1699/1/Momentum3.pdf|date=2017-03-09}}.</ref>[[File:Frans Hals - Portret van René Descartes (cropped)2.jpg|alt=Portrait of René Descartes|thumb|153x153px|René Descartes<br/>(1596–1650)]]{{Blockquote|text=If x is twice the size of y, and is moving half as fast, then there's the same amount of motion in each.|author=|title=|source=}}{{Blockquote|text=[God] created matter, along with its motion ... merely by letting things run their course, he preserves the same amount of motion ... as he put there in the beginning.}}

This should not be read as a statement of the modern law of [[conservation of momentum]], since Descartes had no concept of mass as distinct from weight and size. (The concept of mass, as distinct from weight, was introduced by Newton in 1686.)<ref>{{Cite book |last=Newton |first=I |url=https://books.google.com/books?id=Tm0FAAAAQAAJ&pg=PP13 |title=The mathematical principles of natural philosophy |publisher=Printed for Benjamin Motte |year=1729 |pages=1–2 |translator-last=Motte |translator-first=A. |orig-date=Original work published 1686}}</ref> More important, he believed that it is speed rather than velocity that is conserved. So for Descartes, if a moving object were to bounce off a surface, changing its direction but not its speed, there would be no change in its quantity of motion.<ref>{{Cite book |last=Garber |first=Daniel |title=The Cambridge Companion to Descartes |date=1992 |publisher=Cambridge University Press |isbn=978-0-521-36696-0 |editor=John Cottingham |place=Cambridge |pages=310–319 |chapter=Descartes' Physics}}</ref><ref>{{cite book |last=Rothman |first=Milton A. |url=https://archive.org/details/discoveringnatur0000roth/page/83 |title=Discovering the natural laws: the experimental basis of physics |date=1989 |publisher=Dover |isbn=978-0-486-26178-2 |edition=2nd |location=New York |pages=[https://archive.org/details/discoveringnatur0000roth/page/83 83–88]}}</ref><ref>{{cite encyclopedia |title=Descartes' Physics |encyclopedia=The Stanford Encyclopedia of Philosophy |url=https://plato.stanford.edu/archives/fall2017/entries/descartes-physics/ |access-date=29 November 2019 |date=Fall 2017 |editor-last1=Zalta |editor-first1=Edward N. |first1=Edward |last1=Slowik}}</ref> [[Galileo]], in his ''[[Two New Sciences]]'' (published in 1638), used the [[Italian language|Italian]] word {{lang|it|impeto}} to similarly describe Descartes's quantity of motion.

==== Christiaan Huygens ====
[[File:Christiaan Huygens-painting (cropped).jpeg|alt=Portrait of Christiaan Huygens|thumb|155x155px|Christiaan Huygens<br/>(1629–1695)]]
In the 1600s, [[Christiaan Huygens]] concluded quite early that [[Cartesian laws of motion|Descartes's laws]] for the elastic collision of two bodies must be wrong, and he formulated the correct laws.<ref>{{cite book |title=The Beginnings of Modern Science |publisher=Basic Books |year=1964 |editor-last=Taton |editor-first=Rene |orig-date=1958}}</ref> An important step was his recognition of the [[Galilean invariance]] of the problems.<ref>Garber and Ayers, pp. 666–667.</ref> His views then took many years to be circulated. He passed them on in person to [[William Brouncker, 2nd Viscount Brouncker|William Brouncker]] and [[Christopher Wren]] in London, in 1661.<ref>Garber and Ayers, p. 689.</ref> What Spinoza wrote to [[Henry Oldenburg]] about them, in 1666 during the [[Second Anglo-Dutch War]], was guarded.<ref name="Israel2001">{{cite book |last=Israel |first=Jonathan I. |url=https://books.google.com/books?id=vMvlEweVPTsC&pg=RA3-PR62 |title=Radical Enlightenment: Philosophy and the Making of Modernity 1650–1750 |date=8 February 2001 |publisher=Oxford University Press |isbn=978-0-19-162287-8 |pages=lxii–lxiii |author-link=Jonathan I. Israel |access-date=11 May 2013}}</ref> Huygens had actually worked them out in a manuscript {{lang|la|De motu corporum ex percussione}} in the period 1652–1656. The war ended in 1667, and Huygens announced his results to the Royal Society in 1668. He published them in the {{lang|fr|[[Journal des sçavans]]}} in 1669.<ref>Dictionary, p. 470.</ref>