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I have read many answers here about gravitational potential energy. Suppose that I am holding a ball 5 meters from the ground and I release it and it falls to the ground. Now i get that it falls because I did work against gravity to lift that ball to the height of 5 meters. But my question is that why did I have to do work against gravity? I mean where does gravity get the energy to pull the ball so that I have to do work to lift it? I hope you get my question.

sammy gerbil
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M.doe
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  • Hi. Take a magnet and a metal; bring them close; where did the two objects get the energy to attract each other? Another example: take something made of cloth and rub it, then you may see a static charge when you bring it close to a metal. Why do the charges attract each other? The same goes with gravity, and with each subject(NewtonianmMechanics, GR, EM, QFT) you may find great efforts of mathematical abstraction that try to shed light to the matter: things interact. – Constantine Black Dec 21 '17 at 20:46
  • Potential duplicate of this – Floris Dec 21 '17 at 21:28

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Think of gravity as a sort of spring, trying to pull two objects together: the ball, and the earth. You are standing between them, trying to push them apart. In doing so, you stretch that spring a bit- putting energy into it- and when you let go, the energy you put into the spring is returned as the spring snaps back (that is, as the ball falls down). the farther apart you move them, the more energy you store in that "spring", and the harder it snaps back when you let go (which means the faster the ball is going when it hits the ground). in this sense and in this particular example, there is no energy that the gravitational field has all by itself; energy is stored in and released from the field when we reposition masses within that field.

niels nielsen
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  • I guess my question is that when I store energy into the ball why does it always get turned into kinetic energy – M.doe Dec 30 '17 at 17:53
  • that's just what happens when you let go of something you lifted in a gravitational field. at the moment you let go of it, gravity is right there, pulling down on the ball even before you let go of it, and so the first thing it does when you let go is it begins accelerating away from your hand because that force is there. the next thing that occurs is conservation of energy, which means that the potential energy that you put into the ball by lifting it is exactly converted into kinetic energy. – niels nielsen Dec 30 '17 at 19:06