I'm saying pick an inertial reference frame in which to look at the problem. You can choose any reference frame you want, but an inertial reference frame simplifies things greatly (the reference frame of the glider is NOT an inertial reference frame because the glider undergoes accelerations). Although arbitrary, the earth-fixed frame can be considered an inertial frame from which to look at the problem (you would be correct to say that an earth-fixed reference frame isn't truly inertial, but I don't think the Coriolis force is significant here). In the earth-fixed reference frame, the "windy glider" gains significant kinetic energy when it turns downwind. Are you saying the "windy glider" doesn't REALLY gain kinetic energy when it turns downwind? If it doesn't really gain kinetic energy then it must gain something else that will cause it to break into little pieces if it hits the ground.

Here's another way of looking at essentially the same problem. Suppose a car accelerates from 0 to 100 mph (on a level surface in the earth fixed reference frame). The kinetic energy that the car gains comes from chemical energy stored in the gas. You could look at this problem from a second reference frame where the car is inially travelling at 100 mph and then accelerates to 200 mph. Viewed from the second reference frame, the car gains more kinetic energy during the acceleration than it did when viewed from the earth-fixed frame (it gains three times as much kinetic energy). In both cases the car should burn the same amount of gas, so where did the extra kinetic energy come from in the second reference frame? The answer is that although the force exerted by the ground on the car is the same in both reference frames, the energy supplied by the the force is different. The energy added to a body by a force acting on it is equal to the force times the amount that the body is displaced by the force. In the second reference frame the force on the car from the ground acts through more displacement (three times the displacement) so it adds more kinetic energy.

In the case of the "no-wind" glider, the unbalanced force that turns the glider through 180 degrees doesn't add ANY energy because the glider is always moving perpendicular to the force. In the case of the "windy glider", there will be a component of the glider's motion that is parallel to the unbalanced turning force (this is easiest to see when the glider has turned through 90 degrees). It is this combination of force and displacement in the direction of the force that adds the energy to the "windy glider".

Consider opposite case where the "windy glider" starts downwind and then turns upwind to a "hover". Where did its kinetic energy go? In this case the unbalanced turning force actually takes energy away because there is a component of the glider's motion that is opposite the force.

By this argument the earth cannot be a reference frame either, since it is doing "turns around a point" and is therefore undergoing nearly constant acceleration (elliptical orbit) - not talking about coriolis. The acceleration the earth undergoes is calculable and is analogous to the acceleration the glider undergoes.

Again, I think we mostly agree, I just want to make the important point that kinetic energy (as is motion) is purely relative. The glider can have simultaneously 100 knots worth of kinetic energy relative to the earth, and zero kinetic energy relative to another glider flying in formation with it.

A turn does require an increase in energy. An increase in total energy applied to the atmosphere is required to turn at a constant speed, which can be measured by the increase in load factor (1/cos(theta)). Induced drag increases, thrust required increases, more fuel is burned, etc.

Reference frames for the purpose of calculating kinetic energy or relative velocities are arbitrary and based on our interest at the moment.

Suggested reading: Relativity, by Albert Einstein, or any of a large number of books on the subject by other authors.

As long as the discussion has become ------- --consider the "vomit comet".
How do you measure the kinetic energies of the people floating inside and bumping into each other .
-I am just a country boy - but to me a craft floating/flying in a moving sea of air, is in exactly the same situation as the people floating in the airplane.
gravity is still working of course but the any forces are only relative to the interior of the cabin - nothing else.
No?

OK, I get it, you can choose any reference frame and the kinetic energy will be different. If your position is that kinetic energy can take on any value you choose, then it's not a very useful quantity. The concept of kinetic energy is only useful if you stick to one reference frame for your analysis. In the reference frame of the earth, the kinetic energy of the "windy glider" went up. I think you agree with that point. I think we also both agree that the extra kinetic energy had to come from somewhere. You are trying to dismiss the question without answering where that energy came from.

next---

Perhaps the energy was always there...all the glider knows is that it is travelling at a constant speed (IAS) The air that the glider is flying in is also moving. The glider has a certain amount of energy due to its speed and weight, which for this scenario is constant. It turns from up wind to down wind. IAS does not change. Gliders energy (MV) does not change. Gliders relative ground speed is now IAS plus wind speed. Add the energy of the mass of air the glider is travelling in to the equation and change your reference frame to the ground and then you can see how the total energy of the glider appears to have increased.
Evan.

Once again, right on the mark

Actually, the relevant volume of the Feynman "Lectures on Physics" is far more entertaining and understandable... Feynman was a much better writer than Einstein, and those books are the best treatment of the concepts of physics I've seen anywhere. Light on math, but very precise on concept.

That wasn't what was said. You don't choose the value, you choose the relationship and then work out the value. And often times it isn't a very useful value as often it doesn't describe the reality at all.

All correct, except the concept that darock cleared up about energy taking on any value I choose. It's of course dependent on the frame in question.

I meant no disrespect by sidestepping your question about where the energy comes from; I did that because I assumed you were trying to use that question to suggest that kinetic energy was "absolute". That was an assumption, and if incorrect, I apologize. In my defense, I did ask directly whether that was what you meant, but I wasn't sure I had received a direct answer. In any case, It sounds as if we all understand each other. I have enjoyed this thread.

The energy comes from the forces that cause the air to move over the surface of the earth (or the earth under the air ): coriolis, local heating (diurnal, thermal, from the sun, and the earth), pressure gradients, etc. You already know all that stuff.

Which hurts more?
Flying into a vertical cliff with no wind, flying into it when it's upwind, or flying into it when it's downwind?

Downwind, of course, because the relevant energy is in the frame that everything is in when it all stops.

I would argue that it doesn't hurt at all, or at least not for very long...

but what if the cliff were a horizontal cliff?
Absurd?
Then the question is valid.

Mesae, you make perfect sense. Another way to look at this in an intuitive way is as follows: You are in a 100kt airspeed glider in no wind conditions. Therefore ground speed and airspeed are the same. You commence a 180 degree turn at 3 degrees a second, and in one minute are heading opposite the way you came. Your ground speed went from 100 one way, to 100 the opposite way. Net change is 200 knots in one minute.

Now, same situation, but flying into a 100 knot headwind. Ground speed is zero. You are motionless over the ground. You now commence a downwind turn at 3 degrees a second, as before. One minute later you are on the reciprocal heading. What was the change in ground speed? Simple, it was zero one way, and a minute later 200 the other way. Net change in one minute is exactly what it was in the no wind situation. 200 knots. Same change in speed relative to the earth in the same time frame.

When you are airborne, there is no wind. Don't believe me? Go up in a hot air balloon in a howling gale, and tell me if your hair will be ruffled.

Everything is relative to ones "own little world"
The universe is supposedly hurtling along at umpteen KPH
Our solar system is also zooming along
the earth is zipping around the sun
In our little airplane, putt putting along in the sky - "our little world" is the moving mass of air we are in -
If we measure our aircraft speed against anything else - it all takes on a new vantage point.
My favorite hot air balloon story is still the one about the guy in a balloon who called down to the ground and asked where he was -
200 ft up in a balloon -obvious answer - but of no value to the guy in the balloon.

Andrew,

You hit the nail on the head!! I could not have said it better myself!!

I believe that a lot of RC pilots have no idea what the difference between AIRspeed and GROUNDspeed is, and that there is no correlation between the two.

Sorry for chiming in late...

I think a better way to illustrate this concept being discussed here is to use a boat on a flowing river (with no wind, only the flowing water) instead of an airplane in invisible air. A lot of people can visualize this scenario in their heads more easily than they can with an airplane. Two different modes of transportation, but the exact same principles of physics...

To show the relative motion relationship, have them magine driving the boat in a circle around a leaf floating in the water... and how their track relative to the bank differs with their track relative to the leaf... I get the "Ahhh, I get it" a lot faster with the boat example than with the airplane. People in general are more familiar with how a boat behaves in water than they are with how an airplane behaves in air... it is a more tangible example for them to grasp. The physics of the 2 are the same.