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Regarding the symmetry argument..... There is a definite symmetry
between Fnet and ma. Knowing Fnet and m, you can predict a exactly.
Knowing a and m, you can predict Fnet exactly. However, we can remove
ourselves one step from the relationship Fnet=ma and discover an
asymmetry. There are many possible combinations of forces that can
make up Fnet. Given Fa, Fb, and Fc acting on a mass, there is only one
possible a. But given the acceleration of a mass, all we can infer
directly is Fnet. We don't know if Fa, Fb, and Fc made up Fnet or if
it was Fd and Fe. Therefore, it would seem a reasonable statement that
the combination of forces Fa, Fb, and Fc caused a given mass to have a
particular acceleration.
So while there is no causality implied in Fnet=ma, one can certainly
make a causality argument between a set of applied forces and a
resultant acceleration.