Well, generally when I refer to folk I'm thinking pared down acoustic guitar, without the twang of country, but there is a certain liltingness to the tone that would separate it in my mind from Jack Johnson. But drawing a line is difficult for me, this is just some shaky border I threw up in my head.
I couldn't call rap folk music, because if you want to use the term to mean a culture's traditional music (which is the tack I'm assuming you're taking) then I would think it needs a longer running heritage than rap's fairly short history and growth. And besides, genre labels are basically a tool to help us classify music and so sort through it easier, it sort of defeats their purpose to label rap as some other name which almost universally brings to mind a completely different form of music.
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