'tis true Silverstrata, Newton says so!
As the stationary mass of the glider is turned through 180 degrees it must be accelerated by 40 knots if it is to stay airborne. This requires an input of energy. So a gentle turn whilst trading height for speed is necessary. To believe that an aircraft is excused
Newton formulated the concept of frames of reference. An objects momentum is dependent on it's velocity which is relative to the frame of reference.
The aerodynamics of an aircraft are dependent on it's velocity relative to the air around it. The most useful frame of reference, therefore, is one in which the wind velocity is zero.
The aircraft has no idea whether the air around it is moving relative to the ground.
Tell me: you are currently on a planet rotating at around 1000mph, orbiting the sun at around 6000mph, and orbiting the galactic centre at around 300 000mph.
What would you say your momentum was?
Answer? The correct answer is, "Relative to what?"
By the way, do the math-what is the difference in acceleration between a glider going from 20 knots north to 20 knots south, and one going from stationary to 40 knots south.
Hint:- this requires a correct understanding of the tem acceleration.
Moment in is not exclusivley relative to the Earths surface or anywhere else.