Originally Posted by
Vessbot
The airlner is flying WEST at 500 knots.
OK, fine, the airliner is flying west, so west is the positive direction of our frame of reference. The velocity of the westbound airliner is +500 knots. The man is walking walking in the east direction , so that's -1 knot. a positive 500 knots, plus a negative 1 knot equals positive 499 knots, the man is still moving west at 499 knots. it doesn't become negative because of the direction he is facing.