# Why is Gravity not a Violation of the Conservation of Energy?

1. Mar 16, 2014

### Neolux

The law of Conservation of Energy states that in a closed system (the entire universe or the Earth and Moon?) the total amount of energy is conserved through time.

The gravitational interaction between the Moon and Earth creates tides. Humans can use these tides to create electrical energy. In the cosmos, stars full of energy are created by gravity pulling together clouds of matter and heating them.

My question is: In both of these instances, it seems to me like energy is being added to the system through gravity. Why is this not a violation of The Law of Conservation of Energy?

Thanks.

2. Mar 16, 2014

### ZapperZ

Staff Emeritus
Background check: are you OK with the electrostatic field not violating law of conservation of energy, i.e you understand why it doesn't? I'm trying to understand why you pick gravity in particular, when, say, the electrostatic field shares almost the same physics/mathematics.

Zz.

3. Mar 16, 2014

### Neolux

I'm sorry, I am not very well-versed in the behavior of electrostatic fields or anything like that, but I do have a general knowledge, and I am just looking for a general answer that satisfies my question.

If such an answer does not exist, I am currently learning physics, so i'm sure i'll get to the answer eventually.

4. Mar 16, 2014

### A.T.

Have you done even the most basic research, like looking up on Wikipedia?
http://en.wikipedia.org/wiki/Tidal_power

5. Mar 16, 2014

### ZapperZ

Staff Emeritus
The one aspect of physics that you should be aware of is that there are many things that look seemingly different, but are actually the same, or have the same type of physical description.

What your question is about is the nature of ALL types of "fields", i.e. I've boiled it down to the most fundamental, most general principle. All types of fields have the ability to "do work", to put it naively. Gravitational field is just one example.

So if you can understand the nature of, say, the classical field, for example, and why it doing work isn't really a violation of the conservation of energy, then you've understood a more general form, rather than just a specific example of gravity.

Zz.

6. Mar 16, 2014

### Neolux

Does this mean that if a system involves a field, it can just gain energy infinitely, if the field has the ability to "do work"?

7. Mar 16, 2014

### ZapperZ

Staff Emeritus
I don't know what you mean by "gain energy infinitely". The energy in the field isn't really "infinite". It is also often not constant either over time, or over distance (remember the 1/r^2 dependence?). Besides, look at our world. Where do you see an object that has an infinite amount of energy?

Zz.

8. Mar 16, 2014

### dauto

Because you're forgetting about gravitational potential energy aren't you?

Simple example: Drop an object. It accelerates gaining Kinetic energy. Does that violate conservation of energy? Only if you forget to include gravitational potential energy.

9. Mar 16, 2014

### Staff: Mentor

In the first case the kinetic energy of the earth and moon are being reduced. In the second case the gravitational potential energy of the clouds of matter are being reduced. In both cases one form of energy increases as another form decreases. Energy is therefore conserved.

10. Mar 17, 2014

### Varun Bhardwaj

I understand you,
your question is "will the gravity of any object will not end ever" or "It is the source of infinite energy" i think so
Answer is "It is not infinite, it will end"
eg. An ancient sword (10000 or very old) turns into dust.

11. Mar 17, 2014

### HomogenousCow

You either have significant issues communicating your point or you do not understand physics at a rigorous and serious level.
A physical field (Electromagnetic or gravitational) is an entity which exists everywhere in space and can carry physical quantities such as energy, momentum and angular momentum (depending on the specific field).
Particles interact with each other through the fields, through this we can define various conservation laws between the fields and the particles.
However, newtonian gravity is an incomplete field theory, in the sense that the field has no time evolution of it's own and simply reacts to the particles.

Electromagnetic fields however do not suffer from this problem and represent the shinning example of a complete dynamical system, one that accounts for both the "motions" of the particles and the fields.

Edit: Did not realize Varun Bhardwaj is not the OP

Last edited: Mar 17, 2014
12. Mar 17, 2014

### Staff: Mentor

You seem to believe that these two questions are opposite. They are not. A gravitational field may have a finite amount of energy and still not ever end.

The dust still has the same mass and therefore the same gravity as the sword.