backward vs rearwards

backward is an adjective but rearwards is not an adjective.

Word	Noun	Adjective	Verb	Adverb
backward	No	Yes	No	Yes
rearwards	No	No	No	Yes

As adverbs, backward and rearwards are synonyms defined as:

Other synonyms of backward include back, backwards, rearward.

backward (adjective)	rearwards (adjective)
directed or facing toward the back or rear
(used of temperament or behavior) marked by a retiring nature
retarded in intellectual development
having made less than normal progress

backward (adverb)	rearwards (adverb)
in a manner or order or direction the reverse of normal	at or to or toward the back or rear
in or to or toward a past time
at or to or toward the back or rear