Finnish, right? I thought so from your accent. You didn't know you had an accent when you wrote, did you?

I have no idea why it happens. Nobody does. I don't think anybody ever will. Energy is something we have decided to define in a certain way. The reason we do so is that when it is measured it always is conserved, which makes it useful in describing certain physical interactions and solving certain kinds of engineering problems. But it is primarily a psychological concept. The concepts you mention of kinetic and potential energy are classical physics, the field has come a long way since then, although those ideas are still useful for pedagoigical purposes.

Move a mass to the top of a building, and DEFINE its potential energy as mass times height times g (g is a number associated with earth's gravitational field, every planet has a differnt g).

Now drop that mass to the ground. At any moment during its fall DEFINE its kinetic energy as 1/2 times its mass times its velocity at that moment squared.

So the potential energy is equal to mgh when the object is at the top of the building, and zero when it hits the ground. The kinetic energy is zero at the top of the building, and 1/2mv**2 when it hits the ground. As it loses potential energy during the fall, it gains kinetic energy as it speeds up. The total energy remains constant.

The SUM of the kinetic and potential energy is always the same at any time during the fall because we picked the equations that way! The energy is dispersed as heat or scattering of the pieces when the object hits the ground.

The question you should be asking yourself is why are those particular numbers and those equations chosen when DEFINING potential and kinetic energy. Its because when we pick those definitions you come up with a magic property of the falling object called the "energy" that is always the same. Knowing that it never changes, and that Kinetic+potential=a constant value, makes calculating falling body problems much easier. Those numbers and equations fall naturally out of the math. For example, that 1/2 term in the kinetic energy comes from adding the velocities at top and bottom of the fall and dividing by two to get the average, because the object speeds up as it falls. This happens because of a calculus procedure called differentiation that occurs in the background and is needed for calkculating quantities that are changing. Sorry, but without the math, you can't talk about this, any more than you can discuss music with a man deaf from birth.

I think that you get into trouble by trying to mysticize this too much. Physics is just bookkeeping, we don't discover physical laws, we design them so they coincide with what we measure.

I HIGHLY recommend you go to "The Mechanical Universe" website at

http://www.learner.org/resources/series42.html

where there are some very informative videos that explain all this very clearly, with very clever animations, artwork, demonstrations and actor recreations. The annimations portray the mathematical concepts graphically so you can skip the calculus.

Unit 6 covers Newton's Laws, but make sure you watch the other 5 that precede it, too.

Good Luck