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Objects have properties. For instance, a book has various properties. It has mass and velocity and height above the floor (let's say it is sitting on a table 1 metre above the ground). It's mass is 1kg and because it is not moving, its velocity is 0.
There's a number we can calculate called potential energy, which in the Earth's gravitational field can be calculated as
PE = mgh
where m is the mass (1kg), g is the acceleration due to gravity (10 m/s/s) and h is the height (1m). PE is just a number (10 joules, in fact) that results from the two attributes of the book and the gravity. There is no stuff.
There's another number called kinetic energy which is calculated as
KE = (1/2)mv
2where m is the mass of the object and v is its velocity. There is not stuff. The kinetic energy of the book is zero because it is at rest on the table.
There is a law called the Conservation of Energy that says if we add up all these energies, we always come to the same number. So the total energy of the book (in Newtonian terms) is
TE = KE + PE = 0 + 10 joules.
If I push the book off the table the PE starts decreasing because the height of the book is getting less and less. The KE is increasing because the book is getting faster and faster. However, the conservation of energy tells us that TE is always 10. Just before the book hits the ground, the PE is pretty much zero since the height is zero. The KE must therefore be 10 which tells us that the velocity must be the square root of 20.
No actual stuff has been converted from one form to another. All that has happened is the height of the book has got less and the speed of it has increased.
After the book has hit the floor, its PE is zero because its height is zero and its KE is zero because it is not moving. However, the impact causes the atoms of the air, book and floor to move about more. This we know because we hear the impact and also, with a good enough thermometer, we can measure that the book, air and floor are all slightly warmer. There are ways of calculating the energy of heat and sound and we find that, if we do, we get the number 10 again. Still, no stuff has been converted from one form to another, all that has happened is that the book's impact with the floor has caused molecules to jiggle about more vigorously.
Energy is just a number.