[rudderboost]

Think of it like a boat on a river.

Think of a fast moving river and a boat tied to the dock. The boat 'feels' like its going as fast as the water- say 5 knots. If you untie the boat the boat floats away and 'feels' likes its going zero, but if someone was on the shore with a radar detector they would say you were doing 5 kts. Next fire the motor up and set it to 5 kts speed and head downstream. The boat would 'feel' 5 kts, but the person on the shore with the detector would read 10kts. If you turnws the boat around and went up stream, the boat would 'feel' 5 kts but the person on the shore would show 0 kts.You'd be sitting still.

A boat (airplane) tied to the dock (ground) is subject to the surrounding water (air) but as soon as the boat (plane) is let loose (airborne) it's now a part of the river (airmass).

An old instructor used to say if you had a balloon and there was an ant on it and it was a windy day, the ant would feel the wind as long as you held the balloon in your hand. The momemt you let the balloon go, the ant says "hey the wind stopped", while in actuality the wind is still blowing, but the balloon and the ant are a part of the airmass floating away.

Same as what Scott is saying.