mass
Mass is an important consideration, not so much because of velocity as because of inertia. Would you rather be struck by a ping pong ball traveling 300 mph, or a car traveling 50mph? Of course, mass and velocity together make for a wicked combination. But the faster something strikes the atmosphere, the more impact the atmosphere has on it. The atnmosphere works much like water. If you dive into the water from ten feet, the impact isn't much. Hit the water traveling 220mph, however, and it's like slamming into concrete. Air works pretty much the same way.
But if something has enough mass, it doesn't have to strike the ground very fast to release a lot of energy. Inertia does the job just fine.
But I wouldn't take the King Arthur myth too seriously. It's a parable, and contains some extrmely valuable lessons. Mostly, it's just a very good story with some very good characters.
And, of course, the Bible says God chooses all leaders, the good, the bad, the wicked, the foolish, all for His own purpose. So Biblically speaking, all kings and all leaders are chosen by God.
Be that as it may, true, false, of indifferent, if I find a stone buried in rock, and if no one else can move it, and I somehow manage it, trust me, I will think something special is going on. I'm not dumb enough to think I'm stronger than a bunch of young men with Conan biceps. That was the thing with King Arthur. It wasn't that he could pull the sword from the stone, it was that no one else could, including grown many with several times his strength. That sort of blows the mold theory, too.
But I wouldn't try faking the science of terminal velocity. Even most high school kids will bust you on this one. It's too basic. For that matter, neglect atmospheric resistance, and I doubt there's an editor in the world you could get the manuscript past.