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Let us consider an example of energy in the Earth's gravitational field.

If we put a ball somewhere in this field, the ball starts to accelerate due to the gravitational force exerted upon it.

Gravitational potential energy (GPE) is converted into kinetic energy. But, why does this GPE seem to exist only when I introduce the ball into the gravitational field? Am I creating this GPE somewhere?

Also, for my second question: If the gravitational field does work in accelerating the ball, then it will have to lose energy in order to do this work. Where does all this required energy come from?

From all this thinking, I come to the conclusion that a field is some sort of wizardy with an infinite amount of energy that can be transferred to objects placed in it. I am confused - I've been taught that energy cannot be created.

Where did the Gravitational Potential Energy (and its subsequent conversion into kinetic energy) come from in this case? And more broadly, where do the other types of potential energy come from after placing appropriately interacting objects in those fields (e.g. electrical, magnetic, weak, and strong fields)?

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    $\begingroup$ While we tell you that you are free to choose any zero of potential energy (and this is true for the purposes of working problems), there is a sense in which the most correct zero is all objects at infinite remove from one another. $\endgroup$ – dmckee Feb 19 '14 at 16:06
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As far as I understand this problem, "GPE seems to exist only when I introduce the ball into the gravitational field" is a correct statement. As You said "You introduce the ball", so first You do the work that is converted to GPE and then GPE does work in accelerating the ball. I don't think there is a way in which a ball can appear somewhere within the Earth's gravitational field, without someone putting it there, thereby putting the required energy in.

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