"Ki", or "Aiki"?
Aiki: good physics...
Ki: no idea...
The way I see it, ki is a process that links mind and body to express power. So the expression of ki is definitely physical, but the mind is involved (which is intangible). I would say that a person with "strong ki" is one who is good at linking or co-ordinating mind and body to express power. So if I were to define ki as the effect perceived by an observer/uke, I would say: ki = power (definitely a physical process, as I believe ki balls and no-touch throws etc are all BS). Anyway, it's easy to get swayed by the long history of mystery and mysticism associated with the concept of ki in martial arts; I personally prefer to see ki as simply a concept that when constrained withing the right models it can be useful for learning and teaching certain skills.
Aiki is the union of two ki of opposing directions. Very difficult to do. Saying any of this stuff is "just physics" is almost an oversimplification as saying "eh you're just using atoms, is that all there is to it really? I already use atoms"