Where does the energy of gravity come from?

[bassplayer142]

Say you were to slingshot around the moon and gain momentum. In so doing you would have gained Energy from gravity. Basically because energy is conserved there has to be some way that the force of gravity "radiating" out from the moon would come in contact with a space shuttle and speed it up. How does this work?

 

[magnetar]

I think that the energy you gained ,come from the "kinetic energy" of the moon!

 

[russ_watters]

Gravity is a static force. There is no energy expended in static forces. Consider a book sitting on a table. It has a certain potential energy defined by its height, but that energy level never changes, so gravity is not doing any work on it.

 

[bassplayer142]

I don't understand. I don't see how it could speed something up without transferring some kind of energy.

 

[russ_watters]

Speeding something up is different (I should have read more carefully/been more descriptive), but in order to drop something you first have to lift it. It is conservative, just like a spring. The total energy is the potential energy plus the kinetic energy and it is always constant (if there is no loss from other factors).

A gravitational slingshot, however, does steal orbital energy. It is a bad (confusing) example for examining why there is no energy generation in gravity.

 

[B.E.M]

Hi bassplayer,
You are asking how does a gravitational slingshot work while conserving energy and momentum?

Look at the 'very simple explanation' at the beginning of this example.
http://en.wikipedia.org/wiki/Gravitational_slingshot

Or you might prefer this example that uses an oncoming locomotive rather than algebra (to some people like me, that is much the same thing ;) )
http://www2.jpl.nasa.gov/basics/grav/primer.html

 

[pedridge]

Gravity is a force, if everything is static, there's no work and no energy required. But take two celestial bodies, say the Moon and the Earth. Gravity keep them revolving around each other but the Moon would fly off tangentially if it were not for gravity. The Moon's course is constanlty being altered by the Earth's gravitational pull. This is not a static situation, the moon is being accelerated, work is being done and the question is... where does the energy come from?

 

[russ_watters]

The Moon's course is constanlty being altered by the Earth's gravitational pull. This is not a static situation, the moon is being accelerated, work is being done and the question is... where does the energy come from?

No.
Work is force * distance. Where is the distance here?

 

[pedridge]

Looking at the Moon's trajectory. It is constantly being accelerated towards the Earth. Relative to the Earth there is no movement, but from the Moon's perspective its course is being bent away from its straight line trajectory and therefore work is being done.
Relative to the center of Earth there is no movement, but in rectalinear space the Moon is being constantly accelerated from a constant speed and course, and that is the distance.
Thanks, for your reply, and I'm not tryiing to be argumentative, this puzzles me.

 

[DaveC426913]

When your spaceship uses the Moon to slingshot, the Moon slows down a very, very small fraction. That is where the energy comes from.

It is analagous to a skater grabbing the bumper of a passing car and pulling himself forward. The car slows down as the skater speeds up.

 

[russ_watters]

Looking at the Moon's trajectory. It is constantly being accelerated towards the Earth. Relative to the Earth there is no movement, but from the Moon's perspective its course is being bent away from its straight line trajectory and therefore work is being done. Relative to the center of Earth there is no movement, but in rectalinear space the Moon is being constantly accelerated from a constant speed and course, and that is the distance.

No. The acceleration is always perpendicular to the direction of motion. It causes no change in the kinetic energy of the moon (caveat: elliptical orbits, but they are conservative too). Consider that once a month the moon gets back to where it started, going the same speed as it was a month ago: exactly the same state/energy. Acceleration perpendicular to the direction of motion does not produce a change in speed.

 

[WackyDumpsite]

Hey pedridge,

I think you're onto something. I stumbled upon this thread when I googled the same question you asked.

I agree that gravity introduces energy into the "closed system" of the Universe. Indeed, Newton claimed he was not attempting to explain where gravity came from or what the driving force behind it was.

Physicists just make up terms like "static force" and "potential energy" in order to get around the "matter/energy is neither created nor destroyed." I think it's clear using the thought experiments you propose that matter continuously creates energy in the form of gravitational force from seemingly nothing.

 

[Matterwave]

The moon actually gains energy in its orbit from the rotation of the Earth and the tidal forces the moon exerts on the Earth.

As such, the moon is very slowly moving away from us, and the Earth's rotation is very slowly slowing down.

Over the course of the Earth's history (4.5 billion years), the Earth day has slowed down from ~6 hours to 24 hours, and the moon has drifted something like 4 times further away (don't quote me on these figures).

The point is, energy is not lost, just transferred.

 

[Blenton]

I can imagine one day the travesty of 'wasting moon energy' as space travel becomes more common. The moons energy is a limited resource!

 

[HallsofIvy]

Hey pedridge,

I think you're onto something. I stumbled upon this thread when I googled the same question you asked.

I agree that gravity introduces energy into the "closed system" of the Universe. Indeed, Newton claimed he was not attempting to explain where gravity came from or what the driving force behind it was.

Newton did NOT say that. He did say he was describing gravity, not explaining it ("non hypothesen fingo"- "I frame no hypotheses") but he certainly never talked about a "driving force" behind gravity. One force does not require another "driving force".

Physicists just make up terms like "static force" and "potential energy" in order to get around the "matter/energy is neither created nor destroyed." I think it's clear using the thought experiments you propose that matter continuously creates energy in the form of gravitational force from seemingly nothing.

Yes, in a sense the different kinds of energy are a "bookkeeping" device but very useful ones. Physicists do not "just make up" such terms any more than accountants "just make up" the numbers they put in the books. Matter does not "create" energy from "seemingly nothing". All that the thought experiments baseplayer142 proposes show is that he (and you) do not understand the physics involved. He, at least, asked for assistance. You just tell him to ignore physicists who "just make up" things!

 

[pedridge]

I'll restate the problem I'm having.
Take any two obvious large objects as an example, the two easiest are the Earth and the Moon, locked in an orbit around each other, by gravity. Simple. The question becomes... as the Moon is not being allowed to take off on a tangential course, it is being constantly being accelerated towards the Earth. Simple, again. The energy required must be supplied from somewhere, and the question is where?
Mass and energy are two sides of the same thing, as there is no other source for the energy required, matter has to be converted to energy, but where is the loss of mass?

 

[Matterwave]

The answer is...there is no energy required for a circular orbit to remain stable. That's the whole point people have been trying to tell you, but you seem to ignore.

Remember P=F dot V

Where P is the power (energy per second) radiated, F is the force, and V is the velocity.

For a circular orbit, F and V are always perpendicular (elliptical orbits also don't radiate power, but it's more complicated). Therefore, F dot V is zero and there is no power radiated!

 

[WackyDumpsite]

The answer is...there is no energy required for a circular orbit to remain stable. That's the whole point people have been trying to tell you, but you seem to ignore.

And that makes no sense which is what we've been trying to tell you. Your equations don't help explain how the Sun is able to keep the Earth moving in a circular manner as opposed to flying straight. Or the Earth and the Moon, etc. To say that no energy is needed to maintain a circular orbit is nonsensical. It obviously requires energy.

Take a table. If I kick the bottom of the table, in outer space it will move in the direction it was kicked. But Earth's gravity is continuously pulling it downward. This is work being done. Work requires energy. There is no matter --> energy conversion that explains this as far as I know, and petridge pointed this out as well. So where does it come from?

I've raised this issue up in a conversation with a Stanford PhD physics professor about this. She didn't know how to answer it and said things like 'we don't know much about gravity' and 'we have this concept of potential energy.' If you have better credentials than that, by all means give a real answer, don't just refer to some equation as proof that you don't need to account for it.

 

[WackyDumpsite]

Newton did NOT say that. He did say he was describing gravity, not explaining it ("non hypothesen fingo"- "I frame no hypotheses") but he certainly never talked about a "driving force" behind gravity. One force does not require another "driving force".

I apologize for getting the quote wrong, or even the subtleties of what he said. But how is "describing it, not explaining it" NOT the same as saying he was saying how it works, not where it comes from? Where what comes from? The energy that gravity is clearing introducing into the system.

 

[DaveC426913]

I apologize for getting the quote wrong, or even the subtleties of what he said. But how is "describing it, not explaining it" NOT the same as saying he was saying how it works, not where it comes from?

Objects fall at 9.8m/s^2. That is describing it without explaining it. It says nothing about how or why objects fall at that rate.

 

[Matterwave]

And that makes no sense which is what we've been trying to tell you. Your equations don't help explain how the Sun is able to keep the Earth moving in a circular manner as opposed to flying straight. Or the Earth and the Moon, etc. To say that no energy is needed to maintain a circular orbit is nonsensical. It obviously requires energy.

Take a table. If I kick the bottom of the table, in outer space it will move in the direction it was kicked. But Earth's gravity is continuously pulling it downward. This is work being done. Work requires energy. There is no matter --> energy conversion that explains this as far as I know, and petridge pointed this out as well. So where does it come from?

I've raised this issue up in a conversation with a Stanford PhD physics professor about this. She didn't know how to answer it and said things like 'we don't know much about gravity' and 'we have this concept of potential energy.' If you have better credentials than that, by all means give a real answer, don't just refer to some equation as proof that you don't need to account for it.

Perhaps, it would be easier to ask you where do you think the energy goes?

Energy is transferred, not destroyed. So, if it takes Energy to keep the moon in orbit, where is this energy going?

 

[WackyDumpsite]

Perhaps, it would be easier to ask you where do you think the energy goes?

Energy is transferred, not destroyed. So, if it takes Energy to keep the moon in orbit, where is this energy going?

I don't know. Perhaps it dissipates as heat. Where is the energy transferred when the tides move back and forth because of the Moon's gravitational pull?

 

[Matterwave]

Some of that energy is dissipated as heat because there is friction between the oceans and the Earth. Some of that energy is transferred to the moon, which makes it orbit larger as time goes on. (As I mentioned in a previous post)

But there are no frictional forces in the Earth/moon system (to first approximation anyways), so why would energy be dissipating as heat?

 

[WackyDumpsite]

Some of that energy is dissipated as heat because there is friction between the oceans and the Earth. Some of that energy is transferred to the moon, which makes it orbit larger as time goes on. (As I mentioned in a previous post)

But there are no frictional forces in the Earth/moon system (to first approximation anyways), so why would energy be dissipating as heat?

Aha! So you'll concede that there is energy dissipated as heat in the wave/Moon system. Therefore, there is energy coming from somewhere. This is gravitational energy.

 

[Matterwave]

Were we talking about the wave/Moon system? I thought we were talking about keeping the Moon in it's orbit.

The energy from the wave/Moon system comes from the rotation of the Earth (Hence, why the Earth's rotation is slowing down). It's the rotational kinetic energy.

 

[D H]

The energy required must be supplied from somewhere, and the question is where?

What energy?

Caveat: The Moon is slowly receding from the Earth due to tidal interactions (very slowly; 3.8 cm/yr currently) and the Earth-Moon system is losing a *tiny* bit of energy in the form of gravity waves.

These changes are extremely tiny in comparison to the Moon's orbital energy. Ignoring those tiny effects, there is *no* change in energy as the Moon orbits the Earth.

I'll start with a circular orbit. The gravitational potential energy is obviously constant in a circular orbit. While the velocity vector is constantly changing direction in a circular orbit, the magnitude of the velocity vector does not change. As kinetic energy is 1/2mv², kinetic energy is also constant in a circular orbit. For a non-circular orbit, gravitational and kinetic energy do change over the course of an orbit, but the sum of gravitational and kinetic energy remains constant. There is no change in energy as two objects orbit one another.

 

[Livethefire]

I created a similar thread https://www.physicsforums.com/showthread.php?t=369008"


That sorted it out for me.

 

[nelsongeorge6]

i don't know

 

[Don Hicks]

Classical Physics relates to the observable universe. We are aware of gravity's grip and we observe its interaction with our universe. However, we do not know how gravity operates. While we can accurately define gravity's observable effects, we do not understand the nature of gravity.

Think about it. This is a fundamental force that works throughout the universe. Yet, we are unable to describe what or how this is being done. It may be that we are unable to understand gravity. Since we spend our entire life in its grasp, perhaps we are restricted from seeing it for what it is. I do not care to believe this.

Gravity may be a separate form of energy or it may be a part of the energy we already know. The only thing we truly must face is that we do not know what it is. Only by accepting that fact will we begin to believe there are other ideas of what gravity is and what it can do.

Gravity is the only force that we are unable to shield, direct, create, or destroy (use up as in transfer). Gravity is a perpetual force that can be found everywhere and if we could understand anything about it, we would never need another source of energy.

Wow, don’t believe this? OK, consider this, if we could diminish or increase gravity’s effects in a prescribed area, we could create an unequal gravity field. If we could place one side of a balanced flywheel in the field, the flywheel would become unbalance and would begin to turn. One side of the flywheel is actually falling through the heavier side of the gravity field. While the field remains stable, the flywheel will continue to turn as its edge enters the unequaled gravity field. This means that a knowledge of gravity could, at the least, provide us with a constant, unrelenting source of energy anywhere on this planet. And that is only if we knew how to increase or decrease gravity in one small area.

Isn’t this enough encouragement for us to put everything we can into discovering what gravity really is and not just what it does?

 

[Chronos]

The moon moves when an object slingshots around it. Gravity is not a force, it is the curvature of space due to matter.

 

[edpell]

How about this. We have two steel balls attached to each other with a wire. They are spinning around each other in empty space. No gravity involved. The force that causes the acceleration comes from the tension in the wire. The force of the wire of ball1 is equal and opposite the force on ball2. There is no net force.

If you believe the acceleration is doing work then after some time we should see some effect of that work. But nothing changes the temperatures remain the same. The velocities remain the same. So where is the energy going? Work = force (dot product) distance. In this case force and distance are perpendicular so the (dot product) is zero.

 

[Don Hicks]

The moon moves when an object slingshots around it. Gravity is not a force, it is the curvature of space due to matter.

This is a great demonstration of the effects of gravity; however, it does not explain how gravity does what it does.

As to the question of whether gravity is a force, step off the fifth floor of a building at sea level on planet Earth and tell me if there is not a force at play. For that matter, help a buddy move his stuff into an upstairs apartment and tell me what you are fighting as you climb the stairs with his futon.

The real answer is not a definition of what gravity does, but an explanation of how gravity does it. Conquering gravity would liberate the human race.

 

[Don Hicks]

How about this. We have two steel balls attached to each other with a wire. They are spinning around each other in empty space. No gravity involved. The force that causes the acceleration comes from the tension in the wire. The force of the wire of ball1 is equal and opposite the force on ball2. There is no net force.

If you believe the acceleration is doing work then after some time we should see some effect of that work. But nothing changes the temperatures remain the same. The velocities remain the same. So where is the energy going? Work = force (dot product) distance. In this case force and distance are perpendicular so the (dot product) is zero.

Ok, there is no gravity (perceptible gravity) at play, so why is this part of the discussion?

It is a good thought experiment, but the true energy in this example was applied at the beginning when the balls were originally flung in the “orbit” you have created. They will eventually slow down as effects from other objects play on them, but, aside from the centrifugal force that would separate the two if the wire were cut, there appears to be no other forces at play. What am I missing?

 

[DaveC426913]

Ok, there is no gravity (perceptible gravity) at play, so why is this part of the discussion?

It is a good thought experiment, but the true energy in this example was applied at the beginning when the balls were originally flung in the “orbit” you have created. They will eventually slow down as effects from other objects play on them, but, aside from the centrifugal force that would separate the two if the wire were cut, there appears to be no other forces at play. What am I missing?

That's why he's bringing it up. Same arrangement, different cause. It makes it easier to show why gravity is doing more work since it's easy to see that the connecting wire is doing no work.

 

[edpell]

They will eventually slow down as effects from other objects play on them

Newton's first law is still good "objects in motion will stay in motion". There is no friction they just keep going and going...