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Where does the energy come from when two objects gravitate towards each other?

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My Username    1

I was thinking about this, and couldn't think of the answer.

 

Two objects far away from each other accelerate toward each other in space because gravity attracts them to each other. They come together to form one object. When the two objects were far apart, their combined energy was less than the newly formed object has. Where did that energy come from?

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DrP    357

When they were far apart they had 'potential' energy. They come together to be in a lower state of energy which is more favourable. The potential energy is lost as kinetic energy when the objects mover together. When they crash/hit/land - the kinetic energy is converted to heat or sound as they crash into each other. As far as I recall the total amount of energy is conserved.

Edited by DrP
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HallsofIvy    50

I was thinking about this, and couldn't think of the answer.

 

Two objects far away from each other accelerate toward each other in space because gravity attracts them to each other. They come together to form one object. When the two objects were far apart, their combined energy was less than the newly formed object has. Where did that energy come from?

 

Your last statement, "When the two objects were far apart, their combined energy was less than the newly formed object has" is wrong. As Dr. P said, you need to include "potential energy".

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RiceAWay    59

In order to deal with "where does the energy come from" first you have to define what gravity is. And we do not understand it at all. All we can say is that gravity is a fundamental nature of matter.

Edited by RiceAWay
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swansont    6213

In order to deal with "where does the energy come from" first you have to define what gravity is. And we do not understand it at all. All we can say is that gravity is a fundamental nature of matter.

 

 

I disagree. You just have to know how it behaves, and we do have an understanding of that.

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Strange    2490

In order to deal with "where does the energy come from" first you have to define what gravity is. And we do not understand it at all. All we can say is that gravity is a fundamental nature of matter.

We understand gravity at least as well as any other fundamental property such as charge. Possibly better, because we can describe gravity in terms of a more fundamental thing (geometry).

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RiceAWay    59

We understand gravity at least as well as any other fundamental property such as charge. Possibly better, because we can describe gravity in terms of a more fundamental thing (geometry).

WHAT is gravity. Don't cop out trying to tell us how it works. That's been around since Sir Isaac Newton.

 

Einstein described it as a curvature in space/time.

 

Can anyone suggest what is causing a curvature in space/time other than mass? So we're to believe that it's TIME that is causing gravity?

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Mordred    893

Better question what is mass.

 

Definition "Resistance to inertia change" in other words its a kinenatic desciptive.

 

No time doesn't cause curvature either.

 

Spacetime curvature is a geometric system that maps freefall motion in accordance to geodesic Worldlines. Its a mathematical descriptive of relations. Not an entity unto itself.

 

http://www.scienceforums.net/topic/89395-what-is-space-made-of/page-1

 

Energy is simply the "ability to perform work"

 

Stick to the physics definitions without trying to apply an entity quality to terms such as mass, energy, spacetime etc. They are properties

Edited by Mordred

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My Username    1

So then would the mass of the objects when apart include the potential energy? And would the mass of the objects be relative? I mean, if there were an observer traveling with each object, would they both come to the same conclusions about the mass of each object?

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StringJunky    1504

WHAT is gravity. Don't cop out trying to tell us how it works.

 

It is not the job of science to discuss ontology; it describes behaviour. Ontology is a philosophical subject.

 

 

Ontology is the philosophical study of the nature of being, becoming, existence or reality as well as the basic categories of being and their relations. - wiki

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Bender    132

So then would the mass of the objects when apart include the potential energy? And would the mass of the objects be relative? I mean, if there were an observer traveling with each object, would they both come to the same conclusions about the mass of each object?

This is something I've been wondering about as well: can potential gravitational energy also be seen as mass, and if so, what is the reference? Moving them arbitrarily close together, the potential energy can become any arbitrarily large negative figure.

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imatfaal    2478

This is something I've been wondering about as well: can potential gravitational energy also be seen as mass, and if so, what is the reference? Moving them arbitrarily close together, the potential energy can become any arbitrarily large negative figure.

 

An Increase in GPE would only be seen as an increase in the energy of the system - and I guess would increase the inertia of the system; I would see this as an analogue of binding energy in the microscale

 

On the second point - well I would hate to think how much energy it would take to separate two neutron stars in close binary orbit. Obviously, your arbitrary distances cannot be totally arbitrary as we have universal limits of how much mass you can fit within a radius - and denser than that you have merging black holes.

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Bender    132

So what is the actual radius of a black hole? If it is 0, then the GPE of the original matter would be infinite. Even if it is finite but still very small, the GPE of stars and galaxies could be larger than their actual mass (contributing even more to the GPE and so on). What am I missing?

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RiceAWay    59

Better question what is mass.

 

Definition "Resistance to inertia change" in other words its a kinenatic desciptive.

 

No time doesn't cause curvature either.

 

Spacetime curvature is a geometric system that maps freefall motion in accordance to geodesic Worldlines. Its a mathematical descriptive of relations. Not an entity unto itself.

 

http://www.scienceforums.net/topic/89395-what-is-space-made-of/page-1

 

Energy is simply the "ability to perform work"

 

Stick to the physics definitions without trying to apply an entity quality to terms such as mass, energy, spacetime etc. They are properties

We are in total agreement. But the question is "what is gravity" in particular. And to that we have no answer.

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Delta1212    894

We are in total agreement. But the question is "what is gravity" in particular. And to that we have no answer.

We don't have the answer to "what is X?" where X is anything fundamental. And we only have the answer to "What is Y?" where Y is anything else because we can describe Y in terms of a configuration of somethings or things X, which we don't have an answer to.

 

So, fundamentally, we don't know what anything is at all.

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mistermack    36

Re the original question, if the two objects were connected by a stretched spring, then the kinetic energy that they gain as they are pulled together was potential energy, stored in the deformation of the spring, from it's natural shape.

 

If you think of spacetime as a form of spring, then a very similar process is happening.

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Mordred    893

We don't have the answer to "what is X?" where X is anything fundamental. And we only have the answer to "What is Y?" where Y is anything else because we can describe Y in terms of a configuration of somethings or things X, which we don't have an answer to.

 

So, fundamentally, we don't know what anything is at all.

Good answer not sure I could have said it better myself.

 

Lets look at another basic definition.

 

Potential energy: the energy possessed by a body by virtue of its position relative to others

 

So how many objects do you need to define potential energy?

 

Now think about that definition when the second objects undergoes kinetic motion. What happens to the potential energy in going towards or away from the other object?

 

Then equate that back to your conservation laws. Keep in mind the definition of energy. Ability to perform work forget fundamental just focus on the above definitions.

 

The answer everyone should arrive at is that energy doesn't need to come from anywhere. It is a consequence of relations between two or more objects/events/mass etc. This is precisely why energy is defined as a property. In order to define this property you need to define the system.

 

If you have no interaction between two objects/events etc energy=0 Potential or otherwise. Lol mass itself is an interaction.

 

Now lets take the above one step further mass and energy relations... both are literally due to interactions. In other words they are two properties describing interaction between two or more objects/events etc.

 

Now apply time.definition Rate of change or duration. So time isn't fundamental either but a property.

 

Lol how many forum threads did I just kill ? using nothing more than the Physics definitions?

Edited by Mordred
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RiceAWay    59

We don't have the answer to "what is X?" where X is anything fundamental. And we only have the answer to "What is Y?" where Y is anything else because we can describe Y in terms of a configuration of somethings or things X, which we don't have an answer to.

 

So, fundamentally, we don't know what anything is at all.

We know what X does and so we can describe its effects on other things. But that does NOT define things like "gravity" or "electric charge". What IS it that makes an electron negative or a positron positively charged?

 

These are fundamental laws we cannot explain other than in terms of what they do.

 

And so you're close to the truth when you say that we know nothing about nothing at its base.

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It's known that all objects with a certain amount of mass generate gravity. This is because of Einstein's relitivity theory. Look at it like this:

Get at towel or a blanket and stretch it on all four sides. Now put a poolball on the blanket. What you see is that the blanket is bent.

If you put a smaller ball on the blanket you will see that the smaller ball will roll over to the heavier poolball because the blanket is more bent at the poolball's side.

 

The same thing happens in space. All objects, with mass, bend space (and time). So the more mass the object has, the more gravity it will have.

On earth we consider to have 1G, if the earth had 2x the mass it has now it would be 2G of gravity.

 

When two bodies attract eachother, think about the blanket. Space is bent around both bodies and they just "roll" inside eachother's bent area.

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mistermack    36

I've never liked the rubber sheet picture. I don't think it reflects the reality of 3D spacetime distortion.

 

To understand gravity you need to understand what mass is. And what energy is. Since we're not there yet, we have to make do with models that can represent what's happening accurately, so that we can calculate what will happen.

But they are still just models.

 

My own mental picture is that a massive particle is just a huge amount of wave energy, rotating around itself in a tiny spherical ball.

When it's in a gravitational field, spacetime is pulled out of it's natural symmetrical shape, and the ball becomes slightly unsymmetrical, in the direction of the object causing the field. A bit pear-shaped. So the wave isn't following the same curve everywhere any more. And it's the lack of symmetry that makes the whole thing want to accelerate in the direction of the mass causing the gravitational field.

 

That's my mental picture at the moment. It's sure to be bollocks, but it keeps me happy till I hear one I like better.

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Eise    161

I nearly never post youtube links, but this one is worth it:

 

 

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mistermack    36

I like that better than the rubber sheet. It's very good. But it's carefully phrased at the end. That's "HOW" gravity makes things fall.

It describes it well, but it doesn't say "why" things fall.

 

It sort of transfers the question. Instead of "why do things fall?" you now need to know, "why do things want to go "straight" when the orientation of local space is constantly changing? Why don't massive particles follow the orientation of the local space, as it changes with time?

 

So exactly the same question remains unanswered, it's just been transcribed.

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