Technically (physically) it's force (as in SI's F).

Oh that makes sense! I'm thinking back to how people say "On jest silny" as in "He is strong" I wouldn't say "He is force"

Mind you, you changed 'strength' (noun) to 'strong' (adj) in your example.

"He is forceful" if you insist on playing this game. Another derived adjective if forcible but it has a bit more elusive meaning (forcible entry into a building)