slackness vs negligence

slackness and negligence both are nouns.

Word	Noun	Adjective	Verb	Adverb
slackness	Yes	No	No	No
negligence	Yes	No	No	No

As nouns, negligence is a hypernym of slackness; that is, negligence is a word with a broader meaning than slackness:

Other hypernyms of slackness include neglect, neglectfulness.

slackness (noun)	negligence (noun)
the quality of being loose (not taut)	failure to act with the prudence that a reasonable person would exercise under the same circumstances
the quality of being lax and neglectful	the trait of neglecting responsibilities and lacking concern
weakness characterized by a lack of vitality or energy