flop vs cave in

flop and cave in both are nouns.

Word	Noun	Adjective	Verb	Adverb
flop	Yes	No	Yes	Yes
cave in	Yes	No	Yes	No

As verbs, cave in is a hypernym of flop; that is, cave in is a word with a broader meaning than flop:

Other hypernyms of flop include break, collapse, fall in, founder, give, give way.

flop (noun)	cave in (noun)
the act of throwing yourself down; collapse; sink	the sudden collapse of something into a hollow beneath it
a complete failure
someone who is unsuccessful
an arithmetic operation performed on floating-point numbers

flop (verb)	cave in (verb)
fall suddenly and abruptly	break down, literally or metaphorically
fall loosely
fail utterly; collapse

flop (adverb)	cave in (adverb)
with a flopping sound
completely