Inertia is not a unified concept in physics

Newton's first law, known as the inertia law
Any object in the state of rest or in the state of uniform motion, remains at rest or in uniform motion, unless acted upon by an unbalanced force
The inertia law is in conflict with constant spin
Because it constantly changes direction, a spinning object such as a top or the earth is not in uniform motion.
As such constant spinning is not taken care of by the inertia law, even though it is inertia that maintains constant spinning!

Note that friction slows down the spinning of the toy. Similarly due to the moon, which causes the tides, Earth’s spin is slowed down through friction of water flows. Friction though, and not inertia, causes the slowing.
Physics solution: create a mathematical duality
In order to comply with nature, physicists created a new law differing from the inertia law!
The new law involves angular speed, angular momentum and rotational inertia!
Clearly the rotational inertia has nothing to do with the inertia law!
Now there are two drastically differing concepts of inertia in physics.
The one-ness of inertia in reality
We can only imagine the reality of inertia as being one.
As a matter of fact physicists did not invent a new word to label the inertia that maintains constant spin.
Physicists only gave it the rotational adjective; there is only one inertia in Reality.
First Observation
Because there is only one inertia in reality, physics' duality of inertia (inertia law / rotational inertia) departs from the unity of Nature.
And because inertia is at the base of all of physics, physics is now based on two differing pillars, and that seems to compromise the unity of any subsequent buildup no matter how elaborate or clever!

Inertia Fundamental Problem

Besides physics' 2 concepts of inertia, there is a much more troubling problem wedged deep within our very concept of inertia!
In our concept of inertia this other concept of ours, which is time plays no role!
In physics, inertia (as well as mass by the way) exists independently of our concept of time!
Yet since Einstein revelations, we know that space and time make one!
Because mass, and our concept of inertia integral part of mass, exist in space, according to Einstein thoughts, mass and inertia should exist in time too!
Second Observation
The absence of time within our concept of inertia prevents us human beings from understanding Nature.
The very inert characteristic of inertia, which originates in Newton's inertia law, and which appeared plausible up to Einstein, becomes a liability upon the onset of space-time.

To top it all, inertia is a fictitious idea that has no physical collateral

Inertia is not merely a human idea, inertia is a human invention! The bad news is that this inertia invention of ours is an idea that has no physical collateral.
Think of a tree, the idea we have of a tree, which has physical collateral, is a healthy idea.
But the inertia idea that we invented and that does not coincide to any physical entity is potentially dangerous!
Gravimotion's interpretation of Nature sticks to that fundamental precept of physics, that any conclusion must be based on factual reality. As such inertia, which is a mental invention with no physical collateral whatsoever, ends up being disregarded in gravimotion.

Comments and an Innovative Alternative

Even though serious, these controversies do not compromise the respect we have for physics
Physics’ mathematical description of reality is exquisitely accurate, much more than gravimotion and by far.
On the other hand it turns out that gravimotion's interpretation of Nature is a unified interpretation, while the idea we make of physics mathematics is not.
The abstract interpretations, built-up by our brains on top of physics' mathematics, reach definitely a questionable level.
There is at least another interpretation of Nature that eludes physics' dilemmas!
In that interpretation, called gravimotion, uniform motion takes over inertia as follows:

• Because inertia is the cause of uniform motion in physics, and because inertia is fictitious, uniform motion is compelled to be the cause of itself in reality!
  And one way to enter this new world of gravimotion is simply to recognize that motion is self-sufficient!
• Motion self-sufficiency and inertia making one, physics elusive concept of inertia is redesigned in terms of physical motion in gravimotion's interpretation of Nature.

Because everything is in motion along the expansion of the universe, a fact Newton was not aware of, the state of rest is no doubt also fictitious and making abstraction of it, the inertia law can be rewritten:
Any object in motion, remains in the same motion, unless one adds or subtracts motion to it.
Clearly this law is all about motion self-sufficiency, and not at all about inertia!
There is nothing to understand, simply take note of motion self-sufficiency when the law is about motion self-sufficiency!
In a nutshell:

1. In gravimotion's interpretation of Nature, Newton's 1^st law is interpreted literally, in term of motion, as the word inertia doesn't even appear in it.
2. And because motion is the only entity on which rests gravimotion interpretation of Nature, physics' uniform and rotational concepts of inertia become de facto one, through gravimotion's interpretation!
3. Last but not least, fictitious inertia is metamorphosed into real entity that of motion in gravimotion.

Gravimotion is a new, unheard of, and spectacular interpretation of Nature
There is nothing like gravimotion...
And there is nothing to comprehend either. Just see Nature as is!
Once one discovers the gravimotion mechanism, physics' concept of inertia looks clumsy and antiquated...