Where does the energy for gravity come from?

[feyn]

Basically my question is this( and sorry if my formulations are at times a bit sloppy, English is not my first language) :

1. we know energy can neither be created nor destroyed, it can only neconverted from one form into another

2.We know that an object in space, that is moving, will move along a straight line unless acted upon by some kind of force.

3. if I want to change the direction of an object in space, I have to invest some energy to do that

4. The moon moves around the earth, because there is a constant pull from gravity. Very roughly we could say that gravity pulls the moon towards Earth , but by the time the moon would have hit the Earth , the moon has already moved forward enough enough, so that it is now a quarter turn further (yea, i know it the orbit is not really round, but it is not too bad an approximation to understand the basics. I w ill ignore for now the influence of the sun, th e other planets and whatnot else)

5. If the pull of gravity is constant , where does this pull get all the energy from ? I mean the moon is a big heap of rock. If we would have to move it, we would need some serious thrust engine or something, needing one hell of a lot of energy. So where does this energy come from, which the Earth constantly "invests" to force the moon into it's orbit.

6. Another example would be a comet or a meteorite. When it flies towards Earth it constantly accelerates, getting more and more kinetic energy while flying towards the planet, till it hits it often with impressive speed. Again where does this energy come from ? my first point says energy cannot be created, so it Earth loosing energy over time?

7. In the end there can be only 2 solutions , either gravity gets weaker over time, loosing it's energy slowly but surely (which does not really make sense, since afaik that is clearly not what happens) some other kind of energy is constantly transformed into gravitational energy, which begs the questions what that energy that is , if and how it is replenished (and if it not, what happens when it runs out), and how come this transfer is so exact (since gravity does not seem to get weaker or stronger, so the transfer seems to exactly make up for the energy lost all the time, which would it make it quite a curious energy source)I hope my english and my science sucked not too bad to get my point across

 

[rcgldr]

The energy comes from the objects initial position, going all the way back to just after the universe was created (big bang theory). So at some point in time, the moon ended up orbiting the earth. In an ideal case, the total energy consisting of gravitational potential energy and kinetic energy of the Earth and moon is constant. In reality, only angular momentum is conserved and some energy is lost to heat due to tidal interaction. Wiki article about this effect:

http://en.wikipedia.org/wiki/Tidal_acceleration#Angular_momentum_and_energy

 

[Dale]

3. if I want to change the direction of an object in space, I have to invest some energy to do that

This point is not correct. It takes energy to increase speed, but not to change direction. Of course, you can come up with examples where energy is used to turn, but those are examples of inefficiencies where energy is being wasted somewhere.

 

[Khashishi]

What goes up, must come down. Well, not really, but basically, it takes energy to launch something up from the ground, because it must move against the direction that gravity is pulling. The object slows down as it increases height because the energy is converted into gravitational potential energy. Eventually (if it isn't launched with greater than the escape velocity), the object will fall back down and the energy is returned to the object. The gravitational potential energy is converted back into kinetic energy, and you end up with the object coming down with the precisely the same speed as it was launched up with, if nothing got in the way.

Objects in space have a large gravitational potential energy to start with. Where did this initial energy come from? The big bang.

 

[feyn]

Let us get back to my meteorite. If it accelerates towards earth, it gains momentum hence energy, where does that energy come from ? In my naive understandingn it must somehow come from Earth , since Earth is accelerating it

 

[A.T.]

Let us get back to my meteorite. If it accelerates towards earth, it gains momentum hence energy,

It gains kinetic energy, but loses potential energy. The sum of all energies in the system doesn't change.

 

[mathman]

This point is not correct. It takes energy to increase speed, but not to change direction. Of course, you can come up with examples where energy is used to turn, but those are examples of inefficiencies where energy is being wasted somewhere.

Not so. Changing direction requires force, so energy must be used.

 

[phyzguy]

Not so. Changing direction requires force, so energy must be used.

This is incorrect. The rate of doing work on an object (which is the time rate of change of the energy) is F⋅V. In a circular orbit, F and V are perpendicular, so F⋅V = 0. There is a force, and there is an acceleration, but no work is done, and the energy of the orbiting body does not change. Note that DaleSpam said the same thing.

 

[Dale]

Not so. Changing direction requires force, so energy must be used.

No. Not all forces use energy:

http://hyperphysics.phy-astr.gsu.edu/hbase/work2.html
http://en.m.wikipedia.org/wiki/Work_(physics)#Constraint_forces

As phyzguy said if f.v is 0 then the power is zero. You can think in terms of the potential energy. If you stay at the same potential then no energy is used. The force is the gradient of the potential. So if you move perpendicular to the force then you stay at the same potential and no work is done.

 

[crador]

No. Not all forces use energy:

http://hyperphysics.phy-astr.gsu.edu/hbase/work2.html
http://en.m.wikipedia.org/wiki/Work_(physics)#Constraint_forces

As phyzguy said if f.v is 0 then the power is zero. You can think in terms of the potential energy. If you stay at the same potential then no energy is used. The force is the gradient of the potential. So if you move perpendicular to the force then you stay at the same potential and no work is done.

Let's break this down into even more ultra basics, for the newbies. What is the first formula you learn for energy transferred by work: force times distance. I can apply any force I want without doing work, so long as I don't move the object in the direction of the force. In the orbits mentioned the distances between, for example Earth and moon, are fixed. No change in distance, no energy.

 

[whats that]

Basically my question is this( and sorry if my formulations are at times a bit sloppy, English is not my first language) :

1. we know energy can neither be created nor destroyed, it can only neconverted from one form into another

2.We know that an object in space, that is moving, will move along a straight line unless acted upon by some kind of force.

3. if I want to change the direction of an object in space, I have to invest some energy to do that

4. The moon moves around the earth, because there is a constant pull from gravity. Very roughly we could say that gravity pulls the moon towards Earth , but by the time the moon would have hit the Earth , the moon has already moved forward enough enough, so that it is now a quarter turn further (yea, i know it the orbit is not really round, but it is not too bad an approximation to understand the basics. I w ill ignore for now the influence of the sun, th e other planets and whatnot else)

5. If the pull of gravity is constant , where does this pull get all the energy from ? I mean the moon is a ****ing big heap of rock. If we would have to move it, we would need some serious thrust engine or something, needing one hell of a lot of energy. So where does this energy come from, which the Earth constantly "invests" to force the moon into it's orbit.

6. Another example would be a comet or a meteorite. When it flies towards Earth it constantly accelerates, getting more and more kinetic energy while flying towards the planet, till it hits it often with impressive speed. Again where does this energy come from ? my first point says energy cannot be created, so it Earth loosing energy over time?

7. In the end there can be only 2 solutions , either gravity gets weaker over time, loosing it's energy slowly but surely (which does not really make sense, since afaik that is clearly not what happens) some other kind of energy is constantly transformed into gravitational energy, which begs the questions what that energy that is , if and how it is replenished (and if it not, what happens when it runs out), and how come this transfer is so exact (since gravity does not seem to get weaker or stronger, so the transfer seems to exactly make up for the energy lost all the time, which would it make it quite a curious energy source)I hope my english and my science sucked not too bad to get my point across

I don't think gravity uses energy cause according to Newton anything made of mass has its own gravity the more mass the more gravity.as far as I can say it is related to the natural curvature of space. And I bet gravity does not use energy and gravity is not exactly a force

 

[ibis]

A handy way of visualising this is to view gravity as like a spring or elastic band. The potential energy stored in the gravity 'band' between two objects is a result of separating the two objects. You can put this energy in yourself - putting a satellite in orbit using a load of chemical energy rocket fuel, for example. If the satellite falls back to Earth the kinetic energy it acquires in falling is the energy you put into putting it up there to begin with.

In the case of a meteorite, the potential energy in the gravity 'band' is also the result of the original separation of the meteorite and Earth, and that separation work was done by the Big Bang and the expansion of the universe. The "natural" position for everything is as a single point, separating things out from that point required energy, that energy is recovered when the two things fall together under gravity.

 

[phinds]

This point is not correct. It takes energy to increase speed, but not to change direction. Of course, you can come up with examples where energy is used to turn, but those are examples of inefficiencies where energy is being wasted somewhere.

I think it would be more correct to say that it takes energy to CHANGE speed, not just increase it. Either one (increase or decrease of speed) will change an orbit [Dale, I know you know this, I'm just pointing it out for the OP]

 

[jbriggs444]

Increasing speed takes energy. Decreasing speed releases energy. Changing direction does neither.

Of course if you use an inefficient process (like squirting a hose at the front of a super-tanker to slow it down), you may wind up using energy even though none is required.

 

[phinds]

Increasing speed takes energy. Decreasing speed releases energy. Changing direction does neither.

Of course if you use an inefficient process (like squirting a hose at the front of a super-tanker to slow it down), you may wind up using energy even though none is required.

How do you decrease the speed of a spacecraft without expending energy?

 

[jbriggs444]

How do you decrease the speed of a spacecraft without expending energy?

Reverse slingshot, for example.

Agreed that there are there are no efficient ways if you are deep space depending on using reaction mass and fuel that are carried on board.

 

[ibis]

Reverse slingshot, for example.

Agreed that there are there are no efficient ways if you are deep space depending on using reaction mass and fuel that are carried on board.

Strictly speaking, a reverse slingshot only doesn't use the spacecraft 's energy - the work is done by the planet.

 

[jbriggs444]

In a slingshot, energy is gained by the spacecraft at the expense of the planet. In a reverse slingshot, energy is gained by the planet at the expense of the spacecraft . So yes it does use the spacecraft 's energy.

 

[CWatters]

5. If the pull of gravity is constant , where does this pull get all the energy from ? I mean the moon is a ****ing big heap of rock. If we would have to move it, we would need some serious thrust engine or something, needing one hell of a lot of energy. So where does this energy come from, which the Earth constantly "invests" to force the moon into it's orbit.

The moon stays at a constant distance from the Earth (at least for the purposes of this question) so gravity, which only works in that direction, doesn't move the moon. Fridge magnets need no energy source to stay up for the same reason.

 

[Rob Benham]

Fridge magnets need no energy source to stay up for the same reason.

Probably the only time I can recall Richard Feynman getting testy, was when an interviewer implied he should surely be able to say how the magnetic force works. Professor Feynman's reaction was, more or less, that one would have to go back into decades of learning and build up, step by step, on the most fundamental foundations of that facet of science before such a question could be answered.

That was magnetism, and given enough time, he could have answered that question. I feel fairly sure he wouldn't have used gravity, even in the most simplistic of analogies, to describe that answer.

 

[phyzguy]

Probably the only time I can recall Richard Feynman getting testy, was when an interviewer implied he should surely be able to say how the magnetic force works. Professor Feynman's reaction was, more or less, that one would have to go back into decades of learning and build up, step by step, on the most fundamental foundations of that facet of science before such a question could be answered.

That was magnetism, and given enough time, he could have answered that question. I feel fairly sure he wouldn't have used gravity, even in the most simplistic of analogies, to describe that answer.

And the relevance of this reply is ?

CWatters was trying to explain that a force only requires energy if it moves through a distance. This is in reference to the OP's original question about the energy expenditure of keeping the moon in its orbit. A static force requires no energy expenditure, just as supporting a 1 ton weight hanging from a hook requires no energy expenditure. It appears from your response that you missed the point completely.

 

[Rob Benham]

Fridge magnets need no energy source to stay up for the same reason.

Sorry, I supposed it was an overreaction to comparing magnetism with gravity. I take your point about "a force only requires energy if it moves through a distance." but using that comparison, again seems to be describing gravity as a force - and implying similar rules.

Nearly dawn here. Will get to the point tomorrow.

 

[ibis]

In a slingshot, energy is gained by the spacecraft at the expense of the planet. In a reverse slingshot, energy is gained by the planet at the expense of the spacecraft . So yes it does use the spacecraft 's energy.

Not quite - the point is perhaps semantic, but while the reverse slingshot reduces the spacecraft 's kinetic energy, the spacecraft does not "expend energy" to decrease its velocity. Going back to the original question:

How do you decrease the speed of a spacecraft without expending energy?

and your answer:

Reverse slingshot, for example.

If we take a reduction of the spacecraft 's speed as "expending energy", as you've stated it above, then there is of course no way to reduce the spaceraft's speed without expending energy, because a reduction in speed necessarily implies a reduction in the spacecraft 's kinetic energy (unless it gains mass) - reverse slingshot slows the spacecraft down, thus reduces its kinetic energy, thus is "expending energy" if we use your second definition, and is therefore not an answer to the question asked originally, in which you presumably used a different definition.

So you can either define "expending energy" as meaning "applies energy itself to reducing its own velocity" or as "has its energy reduced" - your first answer is correct using your first definition but not your second, while your second answer is correct using your second definition but not your first one.

 

[jbriggs444]

Not quite - the point is perhaps semantic, but while the reverse slingshot reduces the spacecraft 's kinetic energy, the spacecraft does not "expend energy" to decrease its velocity.

I agree. The point is semantic.

The reverse slingshot does not expend energy because there is a surplus of work coming out of the system of interest. I take "expending energy" in the sense of burning fuel or draining batteries -- drawing down free energy reserves. The bulk kinetic energy of a planetary body comes under that heading. If it is reduced, we are "expending energy". If it is increased then we are doing the opposite of "expending energy".

 

[jim meyer]

Gravity is a force and not energy. It is a mystery quality of mass that has never been explained in a logical manner. The tools we currently have are not able to crack the mystery of gravity. Maybe after a few hundred more years of attempting to figure it out man kind will learn what gravity is.

 

[IgorIGP]

I will try to do the best to make my English is acceptable.
I think that the roundabout reasons explaining lack of work in system Earth - the Moon don't answer the main issue. The answer of Rob Benkham is good that at once defines impossibility of understanding of the mechanism of transformation of energy without understanding of the mechanism of the operating force, namely - the gravitation. However when I reflect on similar things, without paradoxes doesn't do. Here example:
Let in the beginning two bodies - Earth and the Moon f.e., be very far from each other and are in rest. Let total mass of system of these two bodies of Mo. Let the system was out of balance and mutual falling of two bodies at each other began. It is clear that the mass of bodies start growing in a reference system of the system's mass enter.
After falling both bodies make one body with the mass center which which remained in the original position and total mass is equal to the sum of rest mass and masses of thermal energy and energy of radiation. That looks like the initial mass of Mo is made by the sum of mass of rest of bodies and mass of their gravitational fields! paradoxically that the mass of a gravitational field of each body isn't summarized to the mass of the body. That is the first paradox. It would seem, from the point of view of formal logic, a contradiction isn't present as to potential energy of two infinitely far bodies there has to correspond some mass. Also it is the mass of gravitational fields.
On the other hand both bodies weaken a field of each other when they are divided by distance. And when they make one body, that field represents the sum of their fields in any point of space. Antispeeches that more powerful field has smaller mass. That is the second paradox.
I think that somewhere in my reasonings there are a logical mistakes. I will be glad to hear opinion of participants of a forum.
Sorry so much of so much "of" ( in my English. But I sincerely hope that the essence is clear.

 

[xxtankmasterx]

I ain't no expert, but my meager knowledge leads me to bring back the good old bowling-ball idea. Basically, according to that model space is like a surface and large objects create dents or depressions, and say the moon is caught in Earth's "dent," what this means is that technically the moon is going "straight," but because of the "dent" it is going in a circular motion, so therefore even though the moon is going in a circle it uses the same amount of energy that it would use going straight.

 

[NascentOxygen]

please continue ...

 

[PeterDonis]

Let in the beginning two bodies - Earth and the Moon f.e., be very far from each other and are in rest. Let total mass of system of these two bodies of Mo. Let the system was out of balance and mutual falling of two bodies at each other began. It is clear that the mass of bodies start growing in a reference system of the system's mass enter.

No, that is not "clear"; it's wrong. You are leaving out gravitational potential energy. The total mass Mo of the system, as measured from very far away, at the instant that the two bodies are at rest relative to each other, is not just the sum of the two rest masses; it's the sum of the two rest masses, plus the gravitational potential energy associated with the interaction between them.

After falling both bodies make one body with the mass center which which remained in the original position and total mass is equal to the sum of rest mass and masses of thermal energy and energy of radiation.

Yes, but that is the same as the original total mass Mo. Consider the process in two steps: first, the two bodies fall together. At the instant just before they collide and form one body, the total mass of the system, as measured from very far away, is still Mo, because, as above, Mo originally included the gravitational potential energy. All that happens when the bodies fall together is that the gravitational potential energy gets converted to the kinetic energy of the two bodies as they fall together.

Second, after the two bodies have collided and formed one body at rest at the center of mass of the system (note that this means the collision is perfectly inelastic), the kinetic energy of the two bodies has been converted to thermal energy and energy of radiation. The total mass of the system, as measured from very far away, is still Mo.

So there is no "paradox" anywhere; you just need to be clear about what is actually included in the total mass of the system, as measured from far away.

 

[Cecil Tomlinson]

When an object is raised in a gravitational field the mass and total energy of the object increase. When an object is lowered in a gravitational field energy is extracted, and the object loses an equivalent amount of mass as it gives up energy. When water flows from a high reservoir to a lower reservoir and a turbine between those reservoirs extracts energy from the water, the extracted energy comes from the water in the form of a reduction in the mass of the water. A hydroelectric dam, similar to a nuclear reactor, is a device for converting mass to energy.

 

[Hellmut1956]

Dear Cecil, as PeterDonis refreshingly correct stated, the energy of 2 bodies falling from a place onto each other having been at rest at the starting point just converts potential energy in kinetic energy. What you write is at least not complete. If a body is raised in afield its potential energy is increased and this energy has to be added to the system from external. I like to call it energy pump! As a consequence when an object "is lowered in a gravitational field" you do not write if an external force is applied, this would mean that energy is added to the system and this energy would add to the potential energy converted in kinetic energy by an object moving down "a field". We see this when we watch a spacecraft coming back to Earth after visiting the space station! Chemical energy is burned to reduce the amount of kinetic energy of the spacecraft while being in orbit with the space station! So if "something lowers an object" in a field moving it from a higher potential energy level to a lower energy level, this would add kinetic energy to the one generated from converting potential energy to kinetic energy!

You are correct when you say that reducing the potential energy in a water reservoir, giving as an example a hydroelectric dam, energy gets converted to kinetic energy embodied in the mass of the water falling from the reservoir to the bottom and that a turbine converts a part of this energy i.e. into electrical energy. Where you are totally wrong is in putting the analogy with a nuclear plant as a mean to convert mass to energy!

A hydroelectric plant converts a part of the kinetic energy resulting from the water loosing potential energy and converting it into kinetic energy which the generator uses to create electrical energy. The mass of the water while "embodying" the kinetic energy does remain unchanged as to its mass.

A nuclear plant works using completely other kind of energies! In a nuclear plant atoms are split releasing energy as radiation, heat being a part of this radiation! The source of this energy comes from the forces in the atom nucleus and in the nucleus the physics there behave by the means that mass and energy are just 2 conditions energy can have. Mass and energy there are equivalent! So a part of the radiation generated radiates with the wavelength of heat and so it heats up the water in the primary circuit. This heated water is used to heat water in the secondary water circuit just to keep radioactive radiation confined. The water is converted to gas which increases its pressure and as a consequence this gas moving to areas in the circuit with less pressure, kinetic energy, moves the generator that delivers the electricity! To reuse this water in the secondary circuit it has to be cooled down as it is well known in the cooling towers you see next to atomic plants. In a certain sense, most of the energy available due to the atomic reactions is converted into vapor which reflects the poor efficiency of such atomic reactors!

this picture being in german is till readable for readers in other languages!

I am studying right now a variation of physics didactics called system physics and that is based on "karlsruher physics course" that uses the concept that you took as an example with the water damn to describe physics by having physical quantities embodied in a volume and of potentiality fields. Fascinating about this is, that all areas of physics, be it the classical physics, or be it quantum and relativistic physics, but also fluids, thermodynamics, chemistry and rotational and translational mechanics can be described using the same algorithmic equations and the concepts of potential energy either flowing freely from a recipient with a higher potential to a one with a lower potential releasing energy as the result of the difference of the potential energies or requiring a "pump" that pumps energy in the system. To take your example of the water damn again. A way to store energy in a public grid is to pump water from a lower reservoir to one with a higher potential energy level. Here the water pump converts electrical energy into potential water energy by transporting water volumes between the reservoirs.

I have taken the picture of the above table to show the 7 physical quantities and their related fields that result in a "potential energy level" of a reservoir! This style of teaching physics, besides the fact it allows to model problems and simulate them using tools available for "system dynamics", a concept developed by the Sloan Institute at MIT in 1956, I love the tool "Berkeley madonna" and the modeling and simulation language "Modelica" that represent powerful tools.

Just to wake your interest, http://www.physikdidaktik.uni-karlsruhe.de/Strategien/Analogien_englisch.html a link to a resource available at http://www.physikdidaktik.uni-karlsruhe.de/Strategien/Umwege_englisch.html website. http://www.physikdidaktik.uni-karlsruhe.de/publication/ejp/kpc_ejp.pdf the link to an interesting pdf archive and here 2 pictures that should be of interest:

This table taken from the above linked pdf archive shows how the same kind of simple algebraic equations apply to fields of physics normally seen as completely separated. if you keep in mind that physics is just a science that generates models that allow to predict and compute events observed in real or that are used to be searched for, then this view points on physics without reinventing the wheel displays similarities between the different fields of physics, just by applying the methodology used to model complex dynamic systems developed by the Sloan Institute of MIT in 1956. I found this to be very interesting perspective, as it enables me to model what i believe I have understood and verify my understanding by analyzing the results of the modeling and simulation!

This picture shows the inelastic collision as an example shown using a fluid picture that is well suited to represent the facts using the didactics of System Physics.

This final picture I want to add just as an appetizer to look into system physics:

I am presenting all this as it allows to look in more detail into the misconceptions presented by Cecil in this thread and at the same time offers an alternative to correctly dwelf into what Cecil and others have been trying to say. As I wrote elsewhere, try to get more into the topic before just presenting wild not organized and even less understood topics being reflected about!

I, myself want to write, that I am an apprentice who is trying to educate himself in the required mathematics and physics, because on one side I always have had an affinity to physics, but I am also studying it because I am learning the skills required to model a subsystem I am having a concept of in my hobby of naval modeling.

 

[Drakkith]

Thread locked, pending moderation.

 

[Drakkith]

This thread is an absolute mess. 90% of the replies don't address the OP's question in any way and are simply arguing over semantics or what gravity 'actually is' (which doesn't help the OP). I remind all members to please stick to the thread topic and not wander off on a tangent about something unrelated. Thread locked.

Feyn, if you would like to start a new thread on this topic, please feel free to do so.