The earth is the base thats moving at 0mph (stationary)
You place a conveyor belt on it, and a plane on that. You add forward thrust to the plane, this acts against the earth, not the belt and begins to move. As it does so the belt begins to move in the opposite direction at the same speed (so negative to earth) net result is the belt moves at double the speed of the plane reference to the plane in the opposite direction.
Plane speed reference to the ground (your geograpical base) + X mph
Plane speed reference to belt 2*X mph
Belt speed reference to ground -X mph
Please don't take this personally but if you dont understand that, I really cannot make it any clearer.
