FORCE is the long one.
The following "rules" obviously assume the vowel is one or the other; obviously they're not intended to apply to flour (or coral, which is LOT for me).
1. If the spelling is ar (generally after w or qu) or aur then it is NORTH.
2. Word-final or is NORTH.
3. If the spelling is oor, oar or our it is FORCE.
4. If the spelling is or before a vowel, including y pronounced as such and silent e, it is FORCE.
5. orC not preceded by a labial (orthographically) is almost always NORTH, except when covered by rule 6. But there are a few exceptions.
6. Where the simple past tense of a verb ends in FORCE by rule 4, and the past participle ends orn(e), it is also FORCE.
7. orC after a labial is complicated. A preceding /p/ seems to particularly encourage FORCE (porch, pork, port, sport), but FORCE also occurs (though certainly not universal; fork and form are NORTH) after /f/ and /v/, and sword is also FORCE.
Caveat: I do seem to have this distinction natively, but some of my formative (that's NORTH, by the way) influences didn't have it, and I'm not sure I picked up the historic distribution entirely reliably, especially as far as those "rule 7" words and the exceptions to "rule 5".