Isaac Newton and the Laws of Motion

QUESTION: Isaac Newton and the Laws of Motion – Was He Wrong?

ANSWER:

In recent years it has become fashionable to say that Isaac Newton's laws of motion contained an error (the error of assumption that mass is a constant), and that this was corrected by Einstein's Theory of Special Relativity. As Peter Beckmann has pointed out in his book, A History of Pi, this error never existed.

In the Principia Isaac Newton writes,

"Lex I. Corpus omne perseverare in statu suo quiescendi vel movendi uniformiter in directum, nisi quatenus illud a viribus impressis cogitur statum suum mutare."

"Lex II. Mutationem motus proportionalem esse vi motrici impressae, & fieri secundum lineam rectam qua vis illa imprimatur."

"Lex III. Actioni contrariam semper & aequalem esse reactionem: sive corporum duorum actiones in se mutuo semper esse aequales & in partes contrarias dirigi."

Isaac Newton and the Laws of Motion – Original Formula was Correct
These are Isaac Newton’s famous three laws of motion. In translation, the second law reads "The change of momentum is proportional to the motive force impressed; and is made in the direction of the right line in which that force is impressed." Newton defines momentum as follows: "The quantity of momentum is the measure of the same, arising from the velocity and quantity of matter conjointly."

Or, in the symbolic terms of Newton's calculus, F = d(mv)/dt Newton did not know whether or not mass was constant, and he was too careful a scientist to assume so by placing it outside the differential. During the next 200 years, physicists assumed, for convenience, that mass was constant and began to write F=ma or F=m dv/dt. It is this later day shortcut which proved to be incorrect, not Isaac Newton's original laws of motion.

Footnotes:
Excerpted and rendered from the “Introduction” to: Observations Upon the Prophecies of Daniel and the Apocalypse of St. John, by Sir Isaac Newton (London, 1733). Reprinted by: The Oregon Institute of Science and Medicine, Cave Junction, Oregon (Copyright September, 1991). All Rights Reserved in the Original.

INTRODUCTION by Arthur B. Robinson, Cave Junction, Oregon (July, 1991) -- By Permission: James Fletcher Baxter, Lewisville, Texas.