The cheap answer is:
The energy produced by sliding evaporates as frictional energy over the ground on which the stone lies, as heat.
Otherwise, you will have to accept a more complex answer, because your question is a key question that the standard model of physics has not yet been able to visually imagine.
The deeper answer is:
This question relates to mass and movement, which should also be known to us as kinetic energy and is simply regarded by me as kinetic energy.But mass is a very abstractly mathematically disguised term, of which few people have a truly visual idea of its causality.
So mass is an effect on the speed and direction of a movement of an object.It is important to understand that such an object consists of several parts, so that at least two parts exist, which form a composite in which these parts move each other orbiting. For assuming they would rest there in the compound, as it associates the term resting mass, that would be wrong. It is therefore not possible, because that network also moves together with the expanding space. And this movement can then appear relatively dormant to another compound.
But please, never lose sight of the fact that there must be at least two objects, with which we can form a common vector.For the Higgs boson, which is now also a component of matter, also forces the realization that matter consists of at least two parts of which at least one part will move at the speed of light. Or would you rather believe that bosons can move more slowly than the speed of light?
Thus, it would only be necessary to clarify what effect the speed of light resists in such a way that from the outside a slower movement of the object occurs, which has not yet been clarified by science.
No easier than that,… even if the standard model of physics can only describe all this in an abstract veiled mathematically, I illustrate my ideas with my following picture.
Matter consists of more than one particle,
moving at the speed of light.
If you now influence the inner parts in such a way, whether it is simply pushing on your part or also by electromagnetic or gravitational fields, then this can change the direction of the inner movements, so that the vectors of the inner parts (the blue arrows) form a different sum in direction and speed.
Because the sum of the speed of several particles at a constant light speed is always smaller than the speed of light. And the direction of movement now determines how fast that will be.
So, especially the direction of movement was mathematically so obscured that no one could imagine it figuratively, but i do with it.
Because the process of the directions of motion certainly also leads to changes in the orbits of the electrons in the atoms of the stone and the components of the soil on which the stone lies.There are collisions of the elements and this can ultimately be considered electrically or also as a thermal process. But it doesn’t matter, because in the end it will be some interacting particle that will answer the question where the energy you were questioning remained.