Originally Posted by
A Squared
Ummm, if a person in an airliner with a velocity of +500 knots is walking toward the tail at 1 knot, his groundspeed is +499 knots, not -499 knots
no, it's -499 because he is facing the tail. If he stops it goes back to -500. If he turns to face the cockpit while stationary it goes from -500 to +500 almost instantly.