Yuras said:
KE depends on observer, but it doesn't depend on coordinates.
I disagree. Usually observers and coordinates are considered to be synonyms. But insofar as they are distinct, it is the coordinate system that determines the KE, not the observer.
For example, say I am driving in a car observing someone throw a ball. I am free to use coordinates where I am at rest, but I am also free to use coordinates where the ground is at rest. The KE of the ball is different in those two coordinate systems even though I am the same observer.
Yuras said:
The same observer will measure they same value for KE in rectangular, spherical, polar or any other coordinates.
This is not true because “any other coordinates” also includes boosts. As I just described the same observer will obtain different versions of the KE of a ball depending on if they are using coordinates where the car is at rest or where the ground is at rest.
Yuras said:
At this point I'm mostly interesting in defending against your point that I invented the whole thing or it's something non-standard. I hope you agree that the source is pretty standard, don't you?
I agree that the source is standard (authoritative even), and I agree that you didn’t invent the concept or the formula.
I only claim that calling that quantity simply “energy” is your personal usage. It does not appear that MTW supports your usage as they carefully qualify the term in their descriptions.
Yuras said:
Oops, it probably sounds a bit passive-aggressive, sorry
No worries at all. My skin is substantially thicker than that (metaphorically speaking).
Yuras said:
I want clarity in this question because otherwise people will continue referring to this conversation claiming that I inverted the whole concept
I agree that the concept is a standard one: the energy measured by some device. It is just your calling it simply “energy” that is non-standard, and the MTW quotes use wording that I prefer.
Yuras said:
I think the "as measured by" part is here for disambiguation.
The disambiguation is important for communication. That is a big part of pedagogy.
Similarly, it is important to disambiguate between “proper time” and “coordinate time”.