# Where does energy in a field come from?

Let us consider for example Earth's gravitational field.

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

I understand the explanation for this : "Gravitational potential energy (GPE) is converted into kinetic energy". But why is it that this GPE seems 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 won't it have to lose energy in order to do all this work? Where does all this energy required 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. But I am perpetually confused because for all my life, I've been taught that energy cannot be created. So where do all these energies come about, really?

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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. –  dmckee Feb 19 '14 at 16:06

## 1 Answer

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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