somersault vs roll over

somersault is a noun but roll over is not a noun.

Word	Noun	Adjective	Verb	Adverb
somersault	Yes	No	Yes	No
roll over	No	No	Yes	No

As verbs, roll over is a hypernym of somersault; that is, roll over is a word with a broader meaning than somersault:

somersault (noun)	roll over (noun)
an acrobatic feat in which the feet roll over the head (either forward or backward) and return

somersault (verb)	roll over (verb)
do a somersault	make a rolling motion or turn
	re-invest (a previous investment) into a similar fund or security
	negociate to repay a loan at a later date for an additional fee