flatness vs inertia

flatness and inertia both are nouns.

Word	Noun	Adjective	Verb	Adverb
flatness	Yes	No	No	No
inertia	Yes	No	No	No

As nouns, inertia is a hypernym of flatness; that is, inertia is a word with a broader meaning than flatness:

Other hypernyms of flatness include inactiveness, inactivity.

flatness (noun)	inertia (noun)
the property of having two dimensions	a disposition to remain inactive or inert
inactivity; showing an unusual lack of energy	(physics) the tendency of a body to maintain its state of rest or uniform motion unless acted upon by an external force
the property of having little or no contrast; lacking highlights or gloss
a deficiency in flavor
a want of animation or brilliance