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Re: Inertia



It should be noted that in the older literature, the word translated as
"Force" isn't necessarily force in the modern sense. It often meant
"property of" or "capacity for" or "tendency to". So we shouldn't confuse
Newton's "Force of inertia" with the F (or -F) in F=ma.

The difference is projected by the speaker. It is usually the case that
the word "force" is associated with some causal agent. The other kind of
force, one which must occur as frequently as the causal kind, is the
caused force, or passive force of reaction, according to the third law.
Rather than distinguish two kinds of forces we now call them both simply
"force". *Vis inertiae" is a force of the latter kind, a force inherent
in an object, measured by its coefficient of inertia, which appears
whenever the object is subjected to a net external force.

My interpretation of this difference in the use of the term "force" (or
its Latin form) in the time of Newton and modern times differs from
Donald's, but mine is specific. Donald's (which is the traditonal one)
simply demurs translation, an attitude that has always puzzled me since
I think that in this instance, at least, the meaning is evident. Newton
was fully aware of the third law and its implications. He was merely
labeling a force much as we do when we speak of the gravitational force
or the centrifugal force, identifying it with its agent. All forces are
ultimately of the same sort, of course.

Leigh