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Re: F=ma



Just a few in sites into the causes of your wondering.

That's an incomplete Freudian slip. You didn't give the URLs!

I thought Newton *replaced* the previous Cartesian construct
of centripetal [sic] force with gravity. Surely the idea of
a centrifugal force would not have supported any theory of
planetary motion. The Tatler, in 1709, its initial year of
publication, describes Newton's construct in this way, a
model for our present day textbooks: "Thus the Tangential
and Centripetal Forces, by their Counter-struggle, make the
Celestial Bodies describe an exact Ellipsis." Newton did not
use "centripetal" in the way it is used by textbooks today.
He meant it in the same sense that we use "radial", so that
it could be applied to noncircular motion as well. (The fact
that the other component referred to here is only orthogonal
to the centripetal component by virtue of being zero I take
on faith.) Newton's centripetal force pointed toward the Sun.

Leigh

(I've copied this to my son, a Cartesian scholar, for comment.
I hope he can set me straight.)