Is gravity an infinite source of energy?

[sophiecentaur]

Some simple, Classical points - no warped space or speed of light arguments - just to get the ancient basic bits straight.
There is always gravitational potential energy between the Earth and Moon in the same way as there is GPE of the water in a reservoir. The reservoir is not moving so you can only use that GPE once - whilst the water falls down through a turbine. However, the Moon's orbit is much slower than the rotation of the Earth and it 'lags behind' the Earth's rotation. This means it is constantly pulling against water, causing a wave to move around the Earth (East to West) once a (iunar) day and for a net flow of ocean water 'backwards'. This force is also pulling the Moon 'forward' so its orbit is constantly increasing by a bit. The flow of water around the oceans (and through your tidal generators) involves Energy loss / transfer to heat etc. but the total Momentum remains constant (basic conservation law). Whether the tide is ebbing or flowing, energy is still transferred.
If this process were allowed to continue to completion, the Moon's orbit would be very large and the Earth's rotation speed would slow down and be the same as the Moon's (new and slower) orbital speed. Then there would be no tidal drag - just two stationary tidal bulges- one towards and one away from the Moon.
Yes, the Sun also has an effect but let's deal with one thing at a time.
Wherever the Moon is in its orbit, its gravitational attraction adds 'vectorially' to the Earth's attraction. Sometimes it adds, sometimes it subtracts and sometimes it adds in another direction. The effect is minute and I don't think it can be measured directly with a balance. But the cumulative effect (coupling) to the movement of the oceans over many cycles produces a very noticeable tidal effect. On a smooth sphere, with no land - so much less friction, the tides would be huge.

"If the Earth were not rotating. . . . " In that case, the tides would occur twice a month and would be enormous, because the water would have plenty of time to flow towards and away from the Moon's position. This would be the scenario described earlier - once the two bodies have achieved synchronism. I wouldn't want to be around then!

 

[Macocio]

If we use the energy (let's say from the ocean caused by the gravitation of our moon), then we would disturb the Earth's natural centre point of gravity and therefore we can not say that gravity is an infinite source of Energy. Newton's law of universal gravitation states this clearly. F = G(m₁m₂/r²)

r is the radius between two masses their centre point of gravity.

 

[sophiecentaur]

There is no "infinite source of energy", whether we're talking about the Moon or Joe Soap's 'perpetual motion machine'. It is a meaningless concept.
Why do people keep looking for loopholes in this argument?

And, despite what they say in the press, the energy we can / could extract, in practice, from astronomical systems is not relevant to the state of the Solar System.

 

[Borek]

And, despite what they say in the press, the energy we can / could extract, in practice, from astronomical systems is not relevant to the state of the Solar System.

Yet

 

[sophiecentaur]

Ever?

 

[sophiecentaur]

Ever? (Dr Strangelove)

 

[Borek]

Wait till we build Dyson sphere.

 

[sophiecentaur]

Materials to be obtained from Travis Perkins no doubt.

 

[Radrook]

What he is asking is whether the object that is the source of the gravity will ever run out of gravitons via using them all up just as a star uses up all its available fuel and eventually stops shining. Hope that clears up the confusion.

 

[sophiecentaur]

Gravitons aren't like that. They are only exchanged when there is some Energy transfer, I think. If there is no change of gravitational Potential then no gravitons are involved.