Hello henra
Originally Posted by
henra
It got nowhere. The only energy that got converted into heat was the 1/2*m*(0,25m/s)^2 during braking on the ground.
And that was
the only Kinetic Energy that it had left, when it crossed the virtual border between the two reference systems - air, and ground.
Originally Posted by henra
With regard to the air the remaining 1/2*m*(30m/s)^2 is still in the glider.
Being fixed on the ground it is effectively being towed through the air at 30m/s by the ground with regard to the air. See my example of the microlight.
Then, when I am standing watching kids playing in the yard, under a breeze of 0.5m/s, I must have a 0.5m/s corresponding Kinetic Energy.... even though I don't move...
The glider at this time does not move, so it has 0 kinetic energy. (relative to the ground, just to be accurate).
Kinetic Energy goes beyond Aerodynamics, and thus, similar examples are abundant in various other fields of the Dynamics, some involving wheels and legs, instead of air, and wings....
For instance the example of a train at a certain speed, and a passenger walking on the train: if the passenger hits an object on the train, his pain is going to be relatively little, when compared with the pain he would have if he hit somehow, an object on the ground - classic comedy films with guys walking on the top of the train, while the train goes under a very low overpass bridge, come to mind.
The passenger Kinetic Energy relative to the train is given by his walking speed, measured by the little pain of hitting an object on the train, while his Kinetic Energy relative to the ground, is augmented by the train's Kinetic Energy, measured by considerable more pain, and possible destruction in the second case..
With that, back to the glider example, the Kinetic Energy corresponding to the 30m/s speed wind, is the air's Kinetic Energy, not the glider's.
The glider on the ground, is like the passenger off the train - no Kinetic Energy from the air, none from the train. Put the glider on the air - push it to make it fly - it's like having the passenger back on the moving train.
All of these examples contain momentum, and inertia aspects, which may add to the fun, or to the confusion....
Originally Posted by henra
The trick is that you need to understand that for an Aircraft once it has left the ground, the movement of the ground below is totally irrelevant for the ability to stay in the air. And it stays irrelevant until it contacts the ground again. In between only its relative movement through the air is relevant.
After these examples it is also clear that it's good to keep the Energy Conservation equation pure, in its General and Universal aspects, which transgress the specifics of the Dynamics fields, like Aerodynamics.