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Sir Isaac Newton
(25 Dec 1642 - 20 Mar 1727)
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Isaac Newton
“A change in motion”
Illustrated Quote - Medium (500 x 250 px)
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Newton wrote his famous three laws of motion in his magnum opus on mechanics, Philosophiæ Naturalis Principia Mathematica. Translated into English, the title is The Principia: Mathematical Principles of Natural Philosophy, familiarly known as simply the Principia. It was first published on 5 Jul 1687, and as the title suggests, it was written in Latin, the language then commonly used by scientists. As it was originally written, the second law of motion appeared in print as:
“Lex II: Mutationem motus proportionalem esse vi motrici impressae, et fieri secundum lineam rectam quâ vis illa imprimitur.”
Newton did not publish a version in English, though various translations have been made since then. The quote as given above is as translated by I. B. Cohen and Anne Whitman (1999). It is very faithful to the original Latin, as seen by comparing with a direct version from Google Translate:
“Law II: The alteration of motion is ever proportional to the motive force impressed; and is made in the right line in which that force is impressed.”
The sense in which Newton meant “change in motion” is what we now call “change in momentum with respect to time.” In a physics class, it is quickly shown how the statement reduces to the familiar equation: F=ma. Because the force, F and the acceleration a are vector quantities, the direction of the resulting acceleration will be the along same straight line as the direction of the impressed force.
After stating his second law, Newton gives further explanation (as translated by Cohen and Whitman):
“If some force generates any motion, twice the force will generate twice the motion, and three times the force will generate three times the motion, whether the force is impressed all at once or successively by degrees. And if the body was previously moving, the new motion (since motion is always in the same direction as the generative force) is added to the original motion if that motion was in the same direction or is subtracted from the original motion if it was in the opposite direction or, if it was in an oblique direction, is combined obliquely and compounded with it according to the directions of both motions.”
- Science Quotes by Sir Isaac Newton.
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- Sir Isaac Newton’s Apple-Tree - debunking the myth, from Historic Ninepins: A Book of Curiosities by John Timbs (1869)
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- Booklist for Isaac Newton.