What is consumed when an object falls down ?

[technician]

The choice of the zero point of potential energy is always arbitrary. The "zero at infinity" convention is every bit as arbitrary and chosen for convenience as are the "zero at sea level" and "zero at ground level" conventions; it is no more special than any other zero point convention.

Nevertheless... It is taken to be the conventional zero of potential rather than any other position... If you did not take it to be so you would have to make it very clear what position you were taking to be the zero of potential, and why you were not using the conventional zero.
I accept that in day to day calculations it hardly matters but when gravitation is concerned it is important to have a convention and to stick to it. It does have a physical meaning regarding work done.

 

[jartsa]

And what is the gravity field giving up, such that we still have conservation of energy? That's a rhetorical question - if you consider the answer you'll probably find the flaw in this statement.

Energy?

After the gravity field has given up all of its energy, falling is not possible anymore in said gravity field. Well that sounds like a problem.

So maybe gravity field also sucks energy from somewhere.

Those who are familiar with the concept "energy at infinity" know that when a boulder falls to the ground, the "energy at infinity" of all objects on the ground decreases.

 

[grev001]

Gravity is Not "Used Up"

When an object acquires kinetic energy in a gravity field, the energy does not appear from somewhere else. It comes from mass, via E=mc². That is the mass of the object itself very slightly reduces and becomes energy (kinetic, heat, sound at impact).

So filling canyons with rocks makes no difference to the energy of the gravity field. It has none, it is geometry, not energy. The Earth may lose energy by radiation etc, that's quite different.

Defining potential energy of of an item by its location in a gravity field is a very convenient fiction for calculations.

 

[Drakkith]

When an object acquires kinetic energy in a gravity field, the energy does not appear from somewhere else. It comes from mass, via E=mc². That is the mass of the object itself very slightly reduces and becomes energy (kinetic, heat, sound at impact).

I'm not sure this is correct. To me it looks like the energy comes from the gravitational potential energy of the system as a whole, not from the mass of the object itself. Potential energy is transformed into kinetic energy and the mass remains the same.

 

[grev001]

I'm not sure this is correct. To me it looks like the energy comes from the gravitational potential energy of the system as a whole, not from the mass of the object itself. Potential energy is transformed into kinetic energy and the mass remains the same.

Thanks, Drakkith, yes, the transform of energy (into e.g. kinetic) is not in doubt, but for me the nature of PE is very doubtful if it is is only position or arrangement. A rock lying on the Earth's surface compared to the rock far away is a system of lower mass - it is a bound system, just as a hydrogen atom has less mass than proton + distant electron. Yes, the mass change is many decimals tiny for objects more easily described as in Newtonian physics!

See "The Physics Teacher", Vol. 41, No. 8, pp. 486–493, November 2003, which supports my view, in asserting "...a change in the PE of a system of interacting objects is real in that it is always accompanied by a change in mass....

(The html version is available at http://scitation.aip.org/journals/doc/PHTEAH-ft/vol_41/iss_8/486_1.html

 

[D H]

See "The Physics Teacher", Vol. 41, No. 8, pp. 486–493, November 2003, which supports my view, in asserting "...a change in the PE of a system of interacting objects is real in that it is always accompanied by a change in mass....

That article does not discuss either fictitious forces. Does the mass of an object change due to a change in centrifugal potential? Of course not. The fictitious centrifugal force is a figment of an observer's imagination. A useful figment, but a figment nonetheless.

Here's the problem: Gravitation is a fictional force in general relativity. You cannot treat gravitational potential energy the same way you treat potential due to real forces.

 

[grev001]

That article does not discuss either fictitious forces. Does the mass of an object change due to a change in centrifugal potential? Of course not. The fictitious centrifugal force is a figment of an observer's imagination. A useful figment, but a figment nonetheless.

Here's the problem: Gravitation is a fictional force in general relativity. You cannot treat gravitational potential energy the same way you treat potential due to real forces.

Thanks indeed. I understand the point about "centrifugal" force. Am I to take it that the energy comes from a different configuration of spacetime? (You can tell I'm not a physicist.)