Where does energy for gravity come from?

[Endercreeper01]

Where does the energy for gravity come from? I know there is gravitational potential energy, but where is it from?

[tormentor]

it is done really clever ,because when you take an object and put it infinitely far away from planet you get (- infinite) potencial energy when object is on planet the potecial energy is 0 so work done by oject is used to rise the potencial energy .

[studiot]

 

because when you take an object and put it infinitely far away from planet you get (- infinite) potencial energy when object is on planet the potecial energy is 0

 

Are you sure?

 

http://en.wikipedia.org/wiki/Gravitational_potential

[swansont]

In Newtonian physics, the energy comes from the gravitational force and the fact that forces can do work when there is displacement. And since you are bound to ask, that force is present because masses interact with each other. We don't know why that is, but we recognize that it happens and study the effects.

 

 

it is done really clever ,because when you take an object and put it infinitely far away from planet you get (- infinite) potencial energy when object is on planet the potecial energy is 0 so work done by oject is used to rise the potencial energy .

 

The choice of zero potential energy is arbitrary, since we are typically interested in the changes in potential energy.

[Endercreeper01]

In Newtonian physics, the energy comes from the gravitational force and the fact that forces can do work when there is displacement. And since you are bound to ask, that force is present because masses interact with each other. We don't know why that is, but we recognize that it happens and study the effects.

 

 

 

The choice of zero potential energy is arbitrary, since we are typically interested in the changes in potential energy.

but what gives mass the energy to attract each other?

[swansont]

but what gives mass the energy to attract each other?

 

As I said, we don't know why the attraction is there.

[Endercreeper01]

 

As I said, we don't know why the attraction is there.

But does this violate conservation of energy? Because just by these 2 masses going closer together is making there be more potential energy. Energy is being created, and energy cannot be created or destroyed. So does this violate conservation of energy?

[uncool]

But does this violate conservation of energy? Because just by these 2 masses going closer together is making there be more potential energy. Energy is being created, and energy cannot be created or destroyed. So does this violate conservation of energy?

Actually, when the masses are closer together, there is less potential energy. And no, it doesn't violate conservation of energy - the lost potential energy is made up for in kinetic energy.

=Uncool-

[studiot]

 

but what gives mass the energy to attract each other?

 

 

This assumes you require energy to generate a force.

 

That is not the case. Forces and energy are different, independent physical quantities. You need to get this very clear before proceeding to study types of energy.

 

A simple example is a brick sitting on a table. The table exerts a force on the brick. No energy is involved.

 

Energy is only involved, as uncool said, when something changes.

Edited August 9, 2013 by studiot

[Endercreeper01]

Actually, when the masses are closer together, there is less potential energy. And no, it doesn't violate conservation of energy - the lost potential energy is made up for in kinetic energy.

=Uncool-

gravitational potential energy u=GMm/r, so the closer they are, then the more potential energy

[swansont]

gravitational potential energy u=GMm/r, so the closer they are, then the more potential energy

 

U = -GMm/r

 

U is zero at infinite separation, and negative for finite separation.

[tormentor]

 

Are you sure?

 

http://en.wikipedia.org/wiki/Gravitational_potential

ops my bad

[studiot]

 

U = -GMm/r

 

U is zero at infinite separation, and negative for finite separation.

 

 

The negative sign is so that as objects approach closer to each other positve work is obtained from the potential energy change.

So a falling stone does work, but it takes work to raise a stone upwards.

 

You didn't reply to my last post noting that it does not take energy to maintain the presence of a force.

You should also note that the potential is inherent in the system ie both the earth and the stone together, along with their positions and motions. It is not a property one part alone.

[Endercreeper01]

 

This assumes you require energy to generate a force.

 

That is not the case. Forces and energy are different, independent physical quantities. You need to get this very clear before proceeding to study types of energy.

 

A simple example is a brick sitting on a table. The table exerts a force on the brick. No energy is involved.

 

Energy is only involved, as uncool said, when something changes.

but the distance between them is changing

[Endercreeper01]

 

This assumes you require energy to generate a force.

 

That is not the case. Forces and energy are different, independent physical quantities. You need to get this very clear before proceeding to study types of energy.

 

A simple example is a brick sitting on a table. The table exerts a force on the brick. No energy is involved.

 

Energy is only involved, as uncool said, when something changes.

but the distance between them is changing

[studiot]

 

but the distance between them is changing

 

Then work is done on or by some agent.

 

Where does the energy for gravity come from? I know there is gravitational potential energy, but where is it from?

 

You have asked a very reasonable question that often puzzles folks and received a relatively short answer from swansont, as he has many calls on his time here.

 

I have tried to fill in some of the gaps but you don't seem to like responding to more than a small part of posts.

I do not wish to indulge in verbal fencing about this subject.

 

I gave you the answer in post#13.

 

The gravitational energy is inherent in the whole system by virtue of the masses and separations of the particles involved, according to the equations being bandied about. It manifests itself as the (gravitational) potential energy of the system.

 

If either the separation or mass or both change then work is done, perhaps on one or more of the masses or perhaps on an outside body.

 

The work done exactly equals the change in gravitational potential energy.

In other words that is the available energy from the change.

 

In order for the system to have arrived at its present state energy must have been input in the past in the form of work by some agent. We do not necessarily know the details of this, only the quantities.

 

So that is where the energy came from.

 

We observe the effects as 'the force of gravity' in Newtonian mechanics, or alternatively the acceleration due to gravity, but as swansont has already noted, we do not know how this force is generated. That is one of the big question curently being attempted by modern physics.

[Endercreeper01]

 

This assumes you require energy to generate a force.

 

That is not the case. Forces and energy are different, independent physical quantities. You need to get this very clear before proceeding to study types

you need energy to make a force do work on an object

 

Then work is done on or by some agent.

 

 

You have asked a very reasonable question that often puzzles folks and received a relatively short answer from swansont, as he has many calls on his time here.

 

I have tried to fill in some of the gaps but you don't seem to like responding to more than a small part of posts.

I do not wish to indulge in verbal fencing about this subject.

 

I gave you the answer in post#13.

 

The gravitational energy is inherent in the whole system by virtue of the masses and separations of the particles involved, according to the equations being bandied about. It manifests itself as the (gravitational) potential energy of the system.

 

If either the separation or mass or both change then work is done, perhaps on one or more of the masses or perhaps on an outside body.

 

The work done exactly equals the change in gravitational potential energy.

In other words that is the available energy from the change.

 

In order for the system to have arrived at its present state energy must have been input in the past in the form of work by some agent. We do not necessarily know the details of this, only the quantities.

 

So that is where the energy came from.

 

We observe the effects as 'the force of gravity' in Newtonian mechanics, or alternatively the acceleration due to gravity, but as swansont has already noted, we do not know how this force is generated. That is one of the big question curently being attempted by modern physics.

oh

And I did reply to your posts

[Toffo]

Energy for falling comes from potential energy.

Potential energy's maximum energy is zero.

When energy is taken from potential energy, potential energy becomes negative.

There's no limit how negative the potential energy can become, so infinite amount of energy can be taken from potential energy.

[michel123456]

 

U = -GMm/r

 

U is zero at infinite separation, and negative for finite separation.

Potential energy is negative energy?

[swansont]

Potential energy is negative energy?

You can define it any way you want, since only changes in PE are important, but the most convenient definition for Newtonian gravity is to set it to zero at infinity. Regardless of the choice of zero, a gravitational system that gets closer together sees a reduction in PE.

[studiot]

The issue of sign seems to have caught several people out so here is the griff.

 

We define PE as swansont has with a negative sign so that as objects approach each other (their separation decreases), the energy change is positive. We want this because this energy change is a measure of the work done.

 

So in the diagram if an object X moves closer to a mass M from position A where the PE is -10 units to a position B where the PE is -20 units the change of PE would be

 

P_A - P_B = -10 - (-20) = +10

 

If we used a positive convention then we would have

 

P_A - P_B = +10 - (+20) = -10

 

The PE v Separation curve looks like the bottom graph with 0 at infinite separation and (- infinity) at zero separation.

[Didymus]

 

This assumes you require energy to generate a force.

 

Energy may not be consumed to produce the force, but what of when that force is directly harnessed to produce energy?

 

Gravity is commonly moves stuff around, and that kinetic energy is frequently made into electricity.... From gravity.

[studiot]

 

Energy may not be consumed to produce the force, but what of when that force is directly harnessed to produce energy?

 

 

 

What of it?

 

Work is done when a force move its point of application and in no other circumstances.

 

So if the system does work (ie transfers energy out) then the point of application of that force moves.

That is some objects move closer together, in the case you mentioned of hydroelectricity, water moves closer to the centre of the earth.

The energy may be replenished in the system if another external agent raises the water again, which is what happens in solar driven evaporation.

Edited August 16, 2013 by studiot

[Didymus]

So, the Sun's energy pulls water into the clouds? Nope. It provides the last bit of heat that leads to most water evaporating.... But gravity pulling more dense gasses down is what causes that water vapor to rise.

 

I was talking about tidal energy. The moon pulls the tides, moving unfathomable masses of water. Plenty of electricity to be generated from nothing but gravity. The sun has nothing to do with it.

 

...could say the same for geothermal energy... Purely gravity driven.

Edited August 17, 2013 by Didymus

[CramBoom]

How do gravitons fit into this?