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Re: Newton's 2nd Law



Sum(F) = dp/dt is relativistically correct for a particle (by definition no
mass change in common modern interpretation). For non-relativistic speeds,
Sum(F) = ma is also correct for a particle.

However, for non-relativistic open systems -- those in which particles can
enter or leave the system -- neither form is always correct. Briefly, the
momentum of a system can change in either or both of two ways: by a net
external force, or because momentum is carried into or out of the system by
particles entering or leaving. There was a lively discussion of this about 30
years ago. See [S. Siegal, Am J. Phys 40, 183 (1972], [M.S. Tiersten, Am. J.
Phys. 37, 82], and references therein. In case you're interested, John Matese
and I published a Lagrangian formalism for open systems as well. [Greene &
Matese, Eur. J. Phys. 2, 103-108 (1981)]

ron greene