# Does gravity generate energy at least once?

## Main Question or Discussion Point

Imagine two stationary planets close together start falling towards each other, they don't orbit or spin, instead they just move closer until they collide, this causes massive amounts of energy to be released in the form of heat and light which radiates away.

So where is this energy coming from if the mass and gravity hasn't changed?

Remember they are the same size, not moving or spinning and only separated by a relatively short distance (like the moon and earth), yet the force of gravity manages to pull them together causing a huge impact which releases a lot of energy, it seems normal but here energy appears to be created and radiated away, nothing has change except the space between them, energy was created from two objects changing location and merging.

## Answers and Replies

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Why should there be radiated heat and light?

If the planets do not impact on each other, the only conversion of energy that exists is from gravitational potential energy to kinetic energy.
If the planets collide, during this process reactions other than gravitational can produce light and heat, but this will change the mass of the planets.

Why should there be radiated heat and light?

If the planets do not impact on each other, the only conversion of energy that exists is from gravitational potential energy to kinetic energy.
If the planets collide, during this process reactions other than gravitational can produce light and heat, but this will change the mass of the planets.
Impacts create a lot of heat and light among other energy releases like kinetic (rocks being flung into space), the point is energy is being created or coming from somewhere but as gravity is a force i didn't think you could turn it into energy, mass isn't undergoing any nuclear fission or fusion as this is a mostly iron and rocky planet. ;)

So how does the mass change exactly, presuming it all stays together as a new bigger planet?

How do you create your light and heat? If you only have gravitational interaction you cannot produce light.
Gravity is force. But it origins from the gravitational potential (at least in the newtonian approximation). So the energy won by moving the planet closer together can be converted to other forms of energy.

I don't get why you're being difficult, you do know asteroid impacts cause massive destruction and energy release don't you? Comparable to hundreds or thousands of nukes depending on the size?

Well imagine a planet vs planet impact, its going to be many many times more powerful, all that energy has to come from somewhere and so if they start off not moving and gravity pulls them together they're not just going to stop when they touch and release no energy because gravity was the one doing the work, they will do what all impacts do and thats release a load of energy regardless, so where is it coming from exactly if not gravity or mass?

The energy was first in their gravitational potential and a moment before the impact all in kinetic energy. So no energy lost or appeared, everything is fine with the laws.

The energy was first in their gravitational potential and a moment before the impact all in kinetic energy. So no energy lost or appeared, everything is fine with the laws.
But they were stationary to begin with so gravity had to do work to create the motion which caused the impact energy to be released.

Potential energy is pretty much imaginary energy, it doesn't really exist so its like creating energy from nothing which i thought was impossible?

ZapperZ
Staff Emeritus
Potential energy is pretty much imaginary energy, it doesn't really exist so its like creating energy from nothing which i thought was impossible?
Potential energy is NOT "imaginary energy". It might be imaginary from YOUR perspective. It isn't imaginary in physics if you've studied it. Just look at a Hamiltonian or Schrodinger equation. Do not confuse what you don't know with what physics is saying.

You also seem to have trouble understand the concept of "energy conversion".

Zz.

But they were stationary to begin with so gravity had to do work to create the motion
Indeed. Gravitational force did work, it raised kinetic energy of planets at expense of potential energy. Sum of those energies is conserved.

which caused the impact energy to be released.
Impact energy is somehow vague term, but that's about right: kinetic energy of planets drives the release of light, heat and destruction and such :)

Potential energy is pretty much imaginary energy, it doesn't really exist so its like creating energy from nothing which i thought was impossible?
Say what?! 8-o
There is nothing imaginary about potential energy, it is as real as kinetic energy.

I believe that a collision between two objects of similar mass in space is unlikely to occur...for, why should either mass attract the other? If their masses are the same and the distance between them is contant, then the gravitational pull between the two bodies should not cause either planet to exit its orbit.
The collsion may exert a tremendous amount of energy...
Assume F=ma, then assume further 1/2mv2. (2 is squared, wtf)
The energy associated with the planets will be kinetic certainly, when the planet (a or b) falls into the others gravitational field. (though it will cause little difference) that kinetic energy would be amplified due to gravitational acceleration. The said collision would be due to the cancellation of momentum. (i.e. planet A or B's inertia) etc.
Like a form of impedance.

If potential energy is real then where is the energy stored?

Also wouldn't this mean that every bit of matter in the universe has potential energy for every other bit of matter, how exactly does that work? Thats a lot of potential energy!

In response to the original post, here is my take on things: imagine the big bang as the start point, matter is created and pushed away from other matter, then you have loads of potential energy so the question for me is where did the energy for the big bang come from, but then that is another matter
My view is possibly nonsense btw.

So where is this energy coming from if the mass and gravity hasn't changed?
You've gotten the basic answer above...gravitational potential energy is converted to kinetic energy which can be released in a collision.

But in your question, "gravity" has changed....it's certaintly NOT a constant....the gravitational attraction, for example, becomes stronger as objects get closer.

Maybe it would help you to think of the kinetic energy released when two cars crash....they typically get crumpled...THAT energy has come from gasoline energy being converted into kinetic energy which is released by energy smooshing metal.

This might help clarify your understanding of negative gravitational potential energy.

http://en.wikipedia.org/wiki/Gravit...ion_where_gravitational_energy_is_negative.3F

ZapperZ
Staff Emeritus
If potential energy is real then where is the energy stored?

Also wouldn't this mean that every bit of matter in the universe has potential energy for every other bit of matter, how exactly does that work? Thats a lot of potential energy!
This is utterly strange. Your criteria, it seems, for an energy to be "real" is that it must be 'stored'? Where did you come up with such a thing? Since when is this a criteria for an energy to be real? Where is a kinetic energy "stored"?

You haven't answered MY question. Have you looked at a Hamiltonian, or the Schrodinger equation, and see the potential energy term? Or do you not have a clue on what they are and are not aware of their importance?

Potential energy for gravity has a dependence upon DISTANCE. It drops as 1/r. So something very far away has very little gravitational potential energy when compared to something very close! So arguing that "every bit of matter in the universe has a potential energy" is a bit strange, considering that many objects are actually very far from one another.

I think you need to learn a little bit more of basic classical mechanics.

Zz.

So things only have potential energy when close enough to another's gravitational field?

Otherwise every big of matter must have potential energy in relation to every other bit of matter, this would mean there's incredible amounts of potential energy in everything, i find that hard to believe.

ZapperZ
Staff Emeritus
So things only have potential energy when close enough to another's gravitational field?
Which part of "drops off as 1/r" did you not understand?

And note, I was describing gravitational potential energy. There are potential energy in springs, electrostatics, etc.. etc.

You really should stop using your own sense of "belief" here, because I'm sensing that there's a lot of basic mechanics that you don't comprehend.

I'm guessing that there will be no more talk of potential energy being "imaginary" from now on.

Zz.

K^2
If potential energy is real then where is the energy stored?
Gravitational field itself has energy. That's where the gravitational potential energy is stored. If you want to look at it from perspective of General Relativity, curvature of space-time is storing energy, not entirely unlike a loaded spring. (In GR, there is no such thing as gravitational field.)

In response to the original post, here is my take on things: imagine the big bang as the start point, matter is created and pushed away from other matter, then you have loads of potential energy so the question for me is where did the energy for the big bang come from, but then that is another matter
My view is possibly nonsense btw.
Another parallel universe.

In all fairness, all objects must have a potential energy relative to another body. However, the tractive forces of the said body in question do decrease significantly over distance. So, those forces can be ignored.

The total amount of energy is the same before and after the colision. Tough to create the system you'll need to create energy.