slow down vs weaken

slow down and weaken both are verbs.

Word	Noun	Adjective	Verb	Adverb
slow down	No	No	Yes	No
weaken	No	No	Yes	No

As verbs, weaken is a hypernym of slow down; that is, weaken is a word with a broader meaning than slow down:

slow down (verb)	weaken (verb)
become less tense, rest, or take one's ease	become weaker
lose velocity; move more slowly	lessen the strength of
become slow or slower	reduce the level or intensity or size or scope of
reduce the speed of	lessen in force or effect
cause to proceed more slowly	destroy property or hinder normal operations