I think we can say that
использовать - to apply (something permanent)
употреблять - to consume, to use something expendable

As for "слово", "сила" etc. they can be treated as expendables as well as permanent. Therefore it is possible both "употребить слово" and "использовать слово". Though there is a difference: "использовать слово" means to use the word for some specific purpose (mentioned in the context), to use it as a tool, while "употребить слово" normally means just to say the word, no matter for what reason.