Re: What is Ki?
There's a story floating around where Tohei was asked the same question by a student on a train trip, to which he picked up a drink can and replied "That is ki".
Terms like "life force" and "energy" are commonly used metaphors to describe ki - which is an abstract "thing", which conveys meaning within its own unique cultural context and usage. In east Asian cosmology, even inanimate things, such as rocks, rivers, mountains and sky have ki, and applying the term "life force" to such inanimate entities is incomprehensible.
If you stand "naturally', let your arms hang down beside you, and breathe naturally, you will notice that your arms will raise and drop slightly in unison with your breathing. That is but one manifestation of ki.
In physical and mechanistic terms, which, as martial artists/hobbyists, we would want to concern ourselves with, ki is primarily motive forces - i.e. that which makes us/them move, or not move (as the case maybe).
But, if "energy" helps you put concrete meaning on an abstract concept, then go for it.