knock down vs knock

knock down is not a noun while knock is a noun.

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

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