Bill's out by the asteroid-belt, where he's been prospecting for minerals - and the metal-detector shows two heavy-metal sources nearby.
Using his jet-pack he zooms off to the first one. When he gets there he finds it's solid gold-ore - very valuable.
Checking his metal-detector he sees the other metal-source is about same weight - but moving away from him at quite a rate.
Therefore Bill thinks he will need more work `magic' to collect that second one.
You see, Bill was reared in the "Newtonian Tribe" down on Earth, and their witch-doctors had trained him to believe that static things only had one small `magic' called inertia, but that moving things had more, a different and bigger magic called momentum.
So, Bill zooms off after the second asteroid.
This one turns out to be solid platinum-ore. Even more valuable.
But, idly checking his metal-detector, he sees the first asteroid (the gold one) is now moving away from him - and at quite a rate.
So Bill then thinks he will need most `magic' to collect the gold one, because now it's moving away from him.
Suddenly Bill felt very small!
He'd just realized something!
When he was by the gold asteroid he'd thought only the platinum one had momentum magic, but then, when he was on the platinum asteroid it seemed that momentum magic belonged only to the gold asteroid.
But nothing had changed - except Bill's point-of-view.
So Bill decided there was just one sort of `magic' - because, as he'd just realized, everything in the Universe is moving anyway, whether he'd thought it was or not!
Further, he opined it was an energy `magic' because it takes work to overcome it.
Later, when he was gleefully moving both asteroids, he opted to stick with the name inertia, because, like with people and their habits (and beliefs), those asteroids seemed to cling to what they were doing already.
In fact, it took the same amount of work to move an asteroid a certain distance - whether you'd believed it had inertia or momentum, or hadn't thought about it at all!
remember from `inertmass' that objects don't easily move, or stop if they're already moving?
It's always known as inertia if not moving, & sometimes called momentum if moving, but it's the same phenomenon
objects tend to carry on doing what they were doing
When Bill was relaxing back home in Moon orbit, he had time to cogitate on the energy `magic' in asteroids (or anything else for that matter).
His earlier discovery meant he no longer believed in straight movement giving extra magic, because everything is moving anyway - the work involved only depends on your own starting point and destination.
But - looking around at the rotating homes of his neighbors - he reckoned there is something that would always demand extra work - `spin'.
So Bill thought he might save the `momentum' word to use for spin magic - although he knew it was only added, rotational `inertia'.
"Well, to describe that extra `spin' energy ..." he said to himself, "...`angular inertia' would be truthful - but it doesn't roll off the tongue."
"No - think we'll label `spin' magic as "angular momentum", which'll also remind me there's always going to be ordinary inertia, plus additional, special directional work with spinning objects."
"And to always question what those witchdoctors say - twice!" he added in his own mind.
Feeling pleased with himself, Bill then reckoned in future he'd be able to figure out all those extra problems to do with orbiting and rotating masses - using no more than a rock or a brick, and maybe a piece of string.
Wonder if he's right?
Reaction of mass held in rotation, or released:
"Centripetal or Centrifugal Force"
Reaction of mass to varying radius of rotation:
"Conservation of Angular Momentum"
Reaction of mass to varying angles of rotation:
Reactions of mass to other movements: