# Where does the energy of gravity come from?

by bassplayer142
Tags: energy, gravity
P: 6
 Quote by HallsofIvy 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.
P: 15,319
 Quote by WackyDumpsite 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.
P: 2,950
 Quote by WackyDumpsite 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?
P: 6
 Quote by Matterwave 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?
 Sci Advisor P: 2,950 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?
P: 6
 Quote by 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?
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.
 Sci Advisor P: 2,950 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.
Mentor
P: 15,201
 Quote by pedridge 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.
 P: 51 I created a similar thread here That sorted it out for me.
 P: 1 i dont know
 P: 3 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?
 Sci Advisor PF Gold P: 9,485 The moon moves when an object slingshots around it. Gravity is not a force, it is the curvature of space due to matter.
 P: 451 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.
P: 3
 Quote by Chronos 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.
P: 3
 Quote by 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.

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?
P: 15,319
 Quote by Don Hicks 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.
P: 451
 Quote by Don Hicks 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....
P: 15,319
 Quote by edpell Newton's first law is still good "objects in motion will stay in motion". There is no friction they just keep going and going....
Not friction - tidal forces. The objects will eventually stop their rotation with their long axis aligned with the closest gravity well.

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