둘 다 오래 지속된, 연장된의 뜻을 가지고 있지만 미묘하게 다른 점이 있다.

prolong

just means, almost literally, "add length" or "make longer".

(Note that indeed it can be used, unusually, by - say - designers or engineers in the sense of making some part longer, like a piece of metal or the like.)

"prolong" is somewhat "value free" - it just means "add length". Whereas

protract

is sort of a "bad thing" (it's somewhat like using - let's say - "spoil" "ruin" "fatten" or the like). It means stretch out, draw out, pull on, tug on.

(Of course, just to confuse things, I guess in unusual situations you could, indeed, also use "protract" to mean literally draw-out; for example "protract the piece of molten steel an additional 7cm...")

I would be inclined to suggest generally using protract at all times in the sense you allude to, as it is sort of the "value-laden" word of the two. (It is the "negative" word.) Leave prolong as a value-free word used by engineers/etc to mean "add length unto".

Thus, you're staging Sound of Music at high school, you might say "let's prolong it 15 minutes." You're simply saying, as a "good" thing, let's add in another song and add 15 minutes to the running time. Use protract in cases where something is ("horribly") being drawn-out.

Of course ............ we have "prolong the agony" as a common phrase - which completely messes up all that I say above!

However it's true that whenever you mean "good" thing (prolong the performance, prolong your experience, prolong the holiday by adding another city, etc) you do indeed use "prolong" as I suggest above.

Protract comes from the Latin "to draw out, to pull" (the same root as for tractor). Hence it suggests that the activity has been artificially stretched, i.e. lengthened but not overall enlarged. One may wish to prolong a pleasurable experience, but not protract it.

**prolonged orgasm