knock vs knock down

knock is a noun but knock down is not a noun.

Word	Noun	Adjective	Verb	Adverb
knock	Yes	No	Yes	No
knock down	No	No	Yes	No

knock (noun)	knock down (noun)
the act of hitting vigorously
negative criticism
a vigorous blow
the sound of knocking (as on a door or in an engine or bearing)
a bad experience

knock (verb)	knock down (verb)
find fault with; express criticism of; point out real or perceived flaws	shatter as if by explosion
knock against with force or violence	knock down with force
rap with the knuckles	cause to come or go down
deliver a sharp blow or push
make light, repeated taps on a surface
sound like a car engine that is firing too early