manpower vs work force

manpower and work force both are nouns.

Word	Noun	Adjective	Verb	Adverb
manpower	Yes	No	No	No
work force	Yes	No	No	No

As nouns, manpower and work force are synonyms defined as:

Other synonyms of manpower include hands, men, workforce.

manpower (noun)	work force (noun)
the force of workers available	the force of workers available