Newtonian Dynamics uses the mathematical models Force, Mass and Acceleration in the 2nd law A = F/M.
The Force and Acceleration models are vectors and are completely specified by a magnitude and direction, Mass is completely specified by a scalar magnitude (of course specification of units, the objects and application points involved are also relevant). Qualifications such as "unstoppable" or "irresistible" only fuzz the mathematical model, so that it might seem to refer to the behavior of physical reality.
I might reword the question as:
In the one dimensional Newtonian 2nd law (A = F/M), what is A if both F and M increase without limit? The answer is that A is undefined unless one specifies just how F and M increase. Physically and mathematically, this limiting process implies some mechanism which controls the increasing F and M values and their relative rate of increase. Of course this unending process is purely mathematical - at any real step in the process (and in the mathematical limit) F and M have definite magnitudes and A = F/M is well defined.
Pedagogically, I would emphasize that the question is about the behavior of the mathematical model – it is not a question about the measureable behavior of material reality..
Bob Sciamanda
Physics, Edinboro Univ of PA (Em)
treborsciamanda@gmail.com
www.sciamanda.com