weak part vs weakness

weak part and weakness both are nouns.

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

As nouns, weakness is a hypernym of weak part; that is, weakness is a word with a broader meaning than weak part:

weak part: a place of especial vulnerability
weakness: the property of lacking physical or mental strength; liability to failure under pressure or stress or strain

weak part (noun)	weakness (noun)
a place of especial vulnerability	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