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Where does gravity get its energy from?

So, I'm taking a high school environmental science class and we just started a unit on energy. We were talking about different types of energy and one was gravitational. I understand the law of conservation of energy and how it works with most types of energy but, from what I can tell, it doesn't seem to apply to gravity. It seems to me that, with everything that falls, there would be less energy going towards earths gravitational field because energy is used to attract the thing that falls and eventually earth will stop attracting objects from a lack of usable energy, but then a large ball of mass wouldn't have gravity, which also doesn't make sense.

Also, it seems that the more energy a black hole uses to alter the course of everything (especially light) would decrease it's pull because of the energy used to pull things in (no force was used to lift an object entering orbit) but it does the opposite. If all the energy comes from lifting the object then what about a rock that has always been at the top of a mountain since the planet was formed? How did gravity get the energy to completely reverse the trajectory of an object that was thrown in the air? In open space if you lift or throw an object it will keep going unless some type of energy changes that.

I understand that the energy from things falling comes from lifting the object but where does the energy come from when an object slingshots around a planet or moon?

Please remember that I am only in high school and use layman's terms!

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  • $\begingroup$ Are you familiar with the concepts of "potential energy" and "kinetic energy"? As an object falls, its potential energy decreases but its kinetic energy increases as its speed increases, then if it hits the ground without bouncing back up, the kinetic energy from falling is converted to kinetic energy of molecules in the air and ground due to sound waves (which involve molecules moving back and forth as seen here) and heat (which involves random motions of molecules) created by the collision. $\endgroup$ – Hypnosifl Jan 9 '15 at 2:47
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I am assuming you don't know enough calculus for the calculus answer to give much meaning to you in answering your question (which would probably require knowledge of multivariable calculus for a sufficient answer).

The phrase "gravitational energy" isn't exactly the right way of putting it. Rather there are only two types of energy, and "gravitational" isn't one of them: The two are potential energy and kinetic energy. Gravity doesn't "have" energy, but rather when one is near a massive body (like on our earth) one finds that, if he or she climbs a hill, and then drops a rock from the top, that by the time it reaches the bottom of the hill we can measure a certain amount of energy upon impact with the ground. And it just so happens that there is this thing called the work energy theorem which states that work (which is force times distance) is equal to the change in the kinetic energy of the body that moved (in our case the rock). Therefore, gravity "has energy" in the sense that when I drop a rock from the top of a hill the kinetic energy at the bottom is different than the kinetic energy it had when I dropped it, which amounts to saying that the rock simply has a different velocity at the bottom than it did at the top. What changed the velocity you ask? Acceleration. What made it accelerate? The gravitational force, because $\vec{F}$=m$\vec{a}$.

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First: Does conservation of energy apply to gravity?

Yes - as you've correctly pointed out, when you drop an object and it falls to Earth, it must be losing gravitational energy. But, the object is now moving - it has kinetic energy. The potential energy of the gravitational field has been converted into kinetic energy, as the principle of conservation of energy requires.

So if your concern is "the gravitational energy is leaving the system", maybe you have not correctly identified the system. Consider the system to be the Earth AND the object. As the object falls, the gravitational potential energy of the system is decreasing (since the object is getting closer to Earth), but the kinetic energy of the system is increasing due to the motion of the rock - and the energy of the entire system is staying constant in time.

On the other hand, it is not really correct to state conservation of energy as "the energy of the system remains constant". It is far more correct to say "the change in energy of the system must equal the transfer of energy across its boundary." This will allow us to understand the conservation of energy of just the rock falling in the gravitational field in the following way:

The system: The rock

Forces on the rock: Gravity

The rock has an external force acting on it - gravity. This force acts over a certain distance, doing a certain amount of work, and adds energy to the system. The system (just the rock, remember), experiences an increase in kinetic energy. So, the positive increase in energy of the system is equal to the transfer of energy across its boundary (in the force of work, in this case).

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Gravity is a force, moving around within it requires energy to overcome that force. Your problem is that you consider gravity to be energy.

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