Hi FlightDetent,
Newton's laws still apply. The inertial system is the spheres, so the physics is there.
I agree, Newton's laws still apply. But you have to be careful to consider what the momentum and energy is relative to.
If the downwind myth were true, then doing a loop in a glider starting into a headwind which matched the flying speed (so ground speed = 0) would be impossible.
What kinetic energy (when ground speed = 0) is being converted into the height gain?
Would you gain more height if you start the loop downwind?