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Quotation from Dialogues Concerning Two New Sciences (Second to Fourth Days) by Galileo Galilei
“any velocity once imparted to a moving body will be rigidly maintained as long as the external causes of acceleration or retardation are removed, a condition which is found only on horizontal planes”
Dialogues Concerning Two New Sciences (Second to Fourth Days), Third Day, scholium after Problem IX, Proposition XXIII, trans. Henry Crew and Alfonso de Salvio (1914)
A statement close to the later principle of inertia, limited by Galileo to motion along a horizontal plane.
Citation: "any velocity once imparted to a moving body will be rigidly maintained as long as the external causes of acceleration or retardation are removed, a condition which is found only on horizontal planes" — Galileo Galilei, Dialogues Concerning Two New Sciences (Second to Fourth Days), Third Day, scholium after Problem IX, Proposition XXIII, trans. Henry Crew and Alfonso de Salvio (1914). (id galileo-two-new-sciences-q8)
