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I would like to know where gravity gets its energy to attract physical bodies?

I know that the law of conservation states that total energy of an isolated system cannot change. So gravity has to be getting its energy from somewhere, or else things like hydropower plants wouldn't be able to turn the power of the falling water into a spinning rotor.

Just to be clear, Lets create an example:

Lets say we have two objects with equal mass close to each other. So gravity does its job and it pulls each other closer, this gets turned into kinetic energy. This is where I'm lost. According to the law of conservation energy can't be created or destroyed and the kinetic energy comes from the gravitational pull so where does the gravitational pull gets its energy.

If that energy isn't being recycled from some where else then that means you have just created energy, therefore breaking the law of conservation.

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  • $\begingroup$ They get it from nowhere, they already HAVE it. It is a "default" energy of the universe if you wish. Overall energy of the universe is constant. $\endgroup$
    – Asphir Dom
    Aug 25, 2013 at 13:52
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    $\begingroup$ Gravity pull has energy by default, but this energy is not infinite as you think. Gravitational pull has finite energy capacity even across infinite distances. You did not create NEW energy because - before moving objects had energy in the form of "passive" potential energy of gravitational field. You should investigate closely the conception of POTENTIAL energy. $\endgroup$
    – Asphir Dom
    Aug 25, 2013 at 14:09
  • $\begingroup$ @AsphirDom Sorry, but I'm not seeing the cycle of energy here. I'm not seeing where the Gravitational potential get it is energy. $\endgroup$
    – Caesar
    Aug 25, 2013 at 14:45
  • $\begingroup$ @Asphir-Dom Total energy may be constant. But Energy can be converted form one type to another. I myself dont understand how rest energy is converted to potential gravitational energy. $\endgroup$
    – eJunior
    Aug 25, 2013 at 15:28
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    $\begingroup$ I know this question was asked a year ago, just wanted to propose 1 thing. On the minutephysics video youtube.com/watch?v=IM630Z8lho8 it states that a common science misconception is that mass affects gravity, when it is actually energy and momentum. I think it is the energy of momentum of the Earth around the sun creating our gravitational field that "adds" energy. Some people answered that the energy of a ball drop on earth came from the energy taken to lift it. What if you dug a deep hole and dropped a ball in? ... $\endgroup$
    – Gᴇᴏᴍᴇᴛᴇʀ
    Sep 6, 2014 at 22:10

8 Answers 8

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According to the conservation of energy, we cannot create or destroy energy, we can only transform it from one form to the other.

So this justifies that gravity doesn't have an infinite source of energy which never runs out! So it must be getting this energy from somewhere else, right?

Let's take the example of a ball dropped from some height. Gravity of the earth pulls it downward, doing work on the ball and giving it kinetic energy. The question you ask is where did it get this energy from?
Go back a step and think about how this ball ended up at such a height? You lifted it up with your arms, and put it on that height. Your arms did work against gravity, spent some energy to put that ball on that height. Where did that spent energy go? This was given to gravity!

When you do work against gravity, you store energy in the gravitational field as gravitational potential energy, which then gravity uses to do work on that object.

In case of hydro power-plants, the sun is giving energy to the water at sea level, to evaporate and rise(in effect doing work against gravity), which ultimately ends up in dams at a higher height, and then falls converting that initial solar energy to electricity!

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    $\begingroup$ Prefect answer, this is EXACTLY what I was looking for. $\endgroup$
    – Caesar
    Aug 25, 2013 at 15:38
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    $\begingroup$ The problem is that gravity has only a potential energy and it depends on its position in relation to other objects. So its 'energy' varies according to reference points, i.e. its not absolute. If you throw a ball with enough speed it will be able to get off Earth gravitational field, and then what happens to its 'stored energy'? The ball can be pulled by some other planet with energy completely different than previously 'stored'. $\endgroup$
    – eJunior
    Aug 25, 2013 at 15:39
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    $\begingroup$ Don't stress on the simplicity of this analogy a lot. It is pretty dumbed down to explain the OP's question simply. There are a lot of other complications and things to consider, like what would happen if the planet blew up? $\endgroup$
    – udiboy1209
    Aug 25, 2013 at 15:49
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    $\begingroup$ I liked your way of explanation.. You made simple thing even more simple.. $\endgroup$
    – Earth is a Spoon
    Mar 15, 2014 at 12:47
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    $\begingroup$ @MartianCactus it can get extremely confusing to look at such specific cases. This is why in physics we deal with transfer of energy, and don't bother figuring out the source of it. If you went looking for the source, you would reach the big bang. This helps us to define our own levels for energy, like I can say that if the two asteroids are infinitely far apart they will have no gravitational potential energy (which then becomes my reference point). After that we can easily calculate the change in potential energy of the asteroids to the point when they are lets say 100KM apart. $\endgroup$
    – udiboy1209
    Feb 23, 2017 at 7:57
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Lets make one thing clear. The phrase "gravity gets its energy" is technically not accurate as "gravity" is a type of interaction or force that exist between any two bodies having mass.

You may be wondering why objects move in gravity, where do they get energy from? If I have understood essence of your question.

Well, every object has its own gravitational field, which has infinite range. To make things simpler to understand, lets imagine a region of space where there are only two bodies. These bodies are in gravitational field of each other and hence possess some energy by the virtue of their relative position. We call it potential energy, as the form of energy they possess has a potential to do some work.

Both these bodies will move towards each other due to gravity and their gravitational potential energy will be converted to kinetic energy. Hence, the energy that objects get while moving in gravitational field gets converted at the expense of their gravitational potential energy.

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  • $\begingroup$ What I'm having hard time understating is hence possess some energy by the virtue of their relative position, where does this play in the law of conservation? $\endgroup$
    – Caesar
    Aug 25, 2013 at 13:35
  • $\begingroup$ Ok, got your point. Let me try a different approach. I am assuming that you understand what is space-time. Space-time is three dimensional and difficult to image, but for sake of understanding our problem, we can assume it as two dimensional. So, in this regard, every object in such space-time will modify and create a region of depression. Now, if you specify a number for every point in the region around the object (which we will call gravitational potential) then this number varies as you move away or near the object. $\endgroup$
    – orionphy
    Aug 25, 2013 at 13:48
  • $\begingroup$ When you place another object in this region, its gravitational potential will be different at different distance from the first object. Hence, by virtue of the position of the second object relative to the first object, it will have different energy. $\endgroup$
    – orionphy
    Aug 25, 2013 at 13:48
  • $\begingroup$ I understand that gravity causes a bend in space, this is why the earth spins around the sun. Like a marble spinning around the inside a hole. But I still don't get where the energy comes from. Please read my edited post. $\endgroup$
    – Caesar
    Aug 25, 2013 at 14:18
  • $\begingroup$ As per my understanding, "Big bang" happened and our universe (known) came into existence. Everything in the beginning was in form for energy, i.e. photons. At certain point of time energy got converted to matter and gravity came into existence. When gravity came to existence the gravitational energy or potential came into existence, since then it is converting to other forms and vice versa. $\endgroup$
    – orionphy
    Aug 25, 2013 at 14:23
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Lets say we have two objects with equal mass close to each other. So gravity does its job and it pulls each other closer, this gets turned into kinetic energy. This is where I'm lost. According to the law of conservation energy can't be created or destroyed and the kinetic energy comes from the gravitational pull so where does the gravitational pull gets its energy.

In Newtonian gravity there exists a potential energy m1m2/r between two bodies , that is where the energy comes from: the fact that the masses were at a distance r gave them gravitational potential energy which is transformed to the kinetic in your example.

So problem number one is reduced to problem number two "how did these two masses get at a distance r"? . And in order to simplify the problem, let us think of the moon and the earth. Where did the moon get its kinetic and potential energy? We are then led to a cosmological model.

The current answer is that the energy was provided at the beginning of time in the Big Bang model of the universe. This model is a General Relativity model, which has a lot of caveats as far as energy conservation goes, ( see the answer by Motl here) but it is true that for flat spaces, where Special Relativity holds, energy conservation holds as usual .

Thus the answer is , after the cataclysmic events of the beginning of time, when the observable universe appeared, it had a dowry of energy in the forms we study in the lab which follows the usual conservation law.

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  • $\begingroup$ I'm slightly confused by this distance r gave them gravitational potential energy how does a measurement r provide energy? $\endgroup$
    – Martin Barker
    Mar 7, 2018 at 16:10
  • $\begingroup$ @MartinBarker It is not the measurement that creates $r$ . $r$ exists and we measure it. For example we measure the distance r of the moon to the earth, and we fit the orbit using newtonian gravitation. This validates the model which has potential energy between two masses of distance $r$ between them as stated above. To bring a satellite at a distanc $r$ we have to give it kinetic energy, which is transformed to potential energy at the orbit . That is the succesful newtonian gravitation theory $\endgroup$
    – anna v
    Mar 7, 2018 at 17:38
  • $\begingroup$ but you have done it in that answers you say distance r which is a measurement of distance not of energy or potential energy, against the first line r exists and we measure it this is what is confusing me. are these 2 different things by the same notation of r? $\endgroup$
    – Martin Barker
    Mar 7, 2018 at 17:59
  • $\begingroup$ @MartinBarker the distance between two objects exists whether we measure it or not. We call it $r$ to describe it mathematically. Potential gravitational energy between masses m_1 and m_2 is given by the V=m_1m_2/r formula, as a functional form. $\endgroup$
    – anna v
    Mar 7, 2018 at 18:03
  • $\begingroup$ the distance between two objects exists whether we measure it or not. yeah I know and whether we measure it or not it's still a measurement. so if I understand this correctly V( ) is the formula for energy from the big bang left in the distance r between the 2 objects? if so how does gravity a force of the mass get the energy that's not already within the 2 masses?Sorry if this is annoying you, I am trying to understand and might be understanding your explanations wrong but as I read them these are the questions coming to mind. $\endgroup$
    – Martin Barker
    Mar 7, 2018 at 18:39
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Falling water in hydropower plant gets its energy from the Sun: the sunrays heat the ocean water, it evaporates into atmosphere, then when the pressure drops or the temperature falls, the water drops on the Earth surface as rain. This rain water accumulates into rivers which are used by the hydropower plants.

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Main part of question -

"Lets say we have two objects with equal mass close to each other. So gravity does its job and it pulls each other closer..."

Answer -

Let us reverse this process. Suppose the two objects are next to each other, then take them apart. You have to spend energy to do that against gravity. Take the objects 5 miles away. Now with your logic, you can say that energy that you spent in moving the object, is lost. But it is not.

Leave the objects on their own and they will come closer again, and will gain KE and they will gain exactly same amount of KE as the amount you spent in separating them. What it tells is that in either case, the energy is not lost, or created, it is converted from one form to another.

When two objects come closer, they gain KE by borrowing it from potential energy of the system. As the KE increases, the potential energy of the system of two objects decreases (actually it increases with negative sign). When you take them apart, you return this potential energy to system.

As a crude example, you can consider gravity as a spring.

In case of gravity, space and the two objects, all three act together to create that attraction. So, gravity borrows the energy from potential energy of the system of two objects.

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The problem here is that on think that gravity between two masses is a force originating from the masses themselves. Even Newton could not accept that idea. He preferred not to make any theory of how that force originated. Lesage suggested that the force was not an attraction between masses but a pushing force coming from outside, forcing the two masses to get closer. The bigger the masses, the more force they receive because one is shielding the force coming from behind it. Newton was not satisfy with that because no one was able to demonstrate it. If the force originate from the matter in the masses, then it might be possible to block some of that force and the 'attraction' will be smaller. A laser beam can block some of the force but the result is the opposite. The 2 masses get closer as if the attraction force is getting stronger. The best explanation of these results is that there is a force coming from every direction of space and that the light ray blocked some of that force; the result is that the 2 masses are pushed toward one another. If only one mass is used, the moving mass will always be pushed toward the light beam, even if the light is enclosed in a steel box. Then one does not have to find where the energy of gravity comes from because the force is coming from all direction of space and is probably caused by all what is moving through space at the speed of light.

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  • $\begingroup$ Newton himself came up with conclusion that gravity is a feature of matter itself. So, your answer is largely irrelevant, IMO. $\endgroup$
    – OmG
    Mar 24, 2019 at 16:52
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As per Newtonts law each thing have energy to attract another thing, beacause in every thing there is potential energy. Due to friction that potential enrgy can convert in another form. for eg. our sirt colour get more dirty, because of friction between neck and colour, in anciant period for generating th fire people used to rubs to white stones on each other. inside of earth there is so much potential energy,and because of heat or pressure or because of any another reason friction come to exists and due to friction earths potential energy converts in gravity (earth get more energy of attraction than another things which are exists on earh)

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The universe is composed of over $90$% dark matter and dark energy. Dark energy comprises about $70$% of this. So far, dark matter and energy remain undetectable and until there are the mechanism that produces gravity, will probably remain a puzzle. If you want to know more in detail you will need to research quantum mechanics and string theory.

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