Re: derivation of Newton's 3rd law from 2nd law

Tom Roberts wrote:

Sure it is specified: "an external force F_ext pushing right on A". It
really doesn't matter when on A the F_ext is applied, but the phrasing
("pushing") implies it is applied to the left side of A (remember all
three objects are rigid). And again there is no need to assume 100N --
any value of F_ext will do. But of course the combination of the masses
and the force determine the acceleration a (his eq. 1).

that's of course and error, since the world we inhabit is fluid, not
rigid. In relativity this thing is obvious, but you have to undrestand.