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Re: inertial forces (definition)



In considering Joel's cautionary note, I recalled that I had quoted
some fragments of Book 3, System of the World, without paying
attention to Newton's introduction to that book, where he mentions
his original intent (and manuscript) was to provide a popular
treatment "that it might be read by many..." but afterwards he
repented, for fear of giving rise to disputes.

He then reduced this book to the form of Propositions and suggests
prior reading of the Principles: if not all of them as being too
demanding, then at least the Definitions, the Laws of Motion, and
the first three sections of the first book.

Taking him at his word, I see that in the opening Definitions of
Book 1, the first mentions mass, the second the motion (momentum)
and the third discusses the "vis insita", the innate force of matter
which
"is a power of resisting, by which every body, as much as in it lies,
continues in its present state, whether it be of rest or of moving
uniformly forwards in a right line......Upon which account, this vis
insita may, by a most significant name, be called inertia
(vis inertiae) or force of inactivity."

So while Newton may not have distinguished inertial from gravitating
masses, he does write of distinct gravitating and inertial forces,
as I have shown here.

Brian

Brian, this *vis insita* acts on the agency which accelerates
the body. It is not an inertial force acting on the body itself.
We are talking about forces which act on the body in question.

If you push on a body it pushes back at you with a force
proportional to its mass; that is the *vis insita* by means of
which the body resists change in its state of motion.

Leigh