weakness vs weak

weakness is a noun but weak is not a noun.

Word	Noun	Adjective	Verb	Adverb
weakness	Yes	No	No	No
weak	No	Yes	No	No

weakness (noun)	weak (noun)
the property of lacking physical or mental strength; liability to failure under pressure or stress or strain
powerlessness revealed by an inability to act
a penchant for something even though it might not be good for you
a flaw or weak point
the condition of being financially weak

weakness (adjective)	weak (adjective)
	wanting in physical strength
	overly diluted; thin and insipid
	tending downward in price
	likely to fail under stress or pressure
	lacking bodily or muscular strength or vitality
	wanting in moral strength, courage, or will; having the attributes of man as opposed to e.g. divine beings
	deficient in magnitude; barely perceptible; lacking clarity or brightness or loudness etc
	not having authority, political strength, or governing power
	(used of verbs) having standard (or regular) inflection
	deficient in intelligence or mental power
	deficient or lacking in some skill
	(used of vowels or syllables) pronounced with little or no stress