weakness vs weak part

weakness and weak part both are nouns.

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

As nouns, weak part is a hyponym of weakness; that is, weak part is a word with a more specific, narrower meaning than weakness:

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

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