inertia vs sluggishness

inertia and sluggishness both are nouns.

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

As nouns, sluggishness is a hyponym of inertia; that is, sluggishness is a word with a more specific, narrower meaning than inertia:

Other hyponyms of inertia include flatness, languor, lethargy, phlegm, restfulness, passiveness, passivity, indolence, laziness.

inertia (noun)	sluggishness (noun)
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 pace of things that move relatively slowly
	a state of comatose torpor (as found in sleeping sickness)