waspy said, "This is definitely not a perpetual motion machine, to class it as such is misleading."
Not at all. The physics of this situation are not subject to cherry-picking which ones, and which half of the cycle to apply them. If the mythical device can produce force without changing any momentum, it can change momentum by focusing that force. Those who accept the former are agreeing that the latter is also true - and they believe they have solved the quest for perpetual motion.
Sorry if this angers you (the red-faced little emoticon says so). This is a good discussion, and you have a darn good understanding of the situation - in fact, I can't figure out where you stand on this, because you flip from an erudite discussion of F=MA into a half backed analysis of half the cycle.