Conservation of Energy
The most important of all physical laws is the law of conservation of energy:
LAW OF CONSERVATION OF ENERGY: Energy may be neither created nor destroyed but only transformed from one form to another.
Motion involves mechanical energy, which has two forms: one is kinetic energy, and it is the energy a body has because of its state of motion; the other form is potential energy, and it is the energy a body has because of its position in a field of force. A stone at the top of a hill, for example, can be said to have energy by virtue of its position in the earth's gravitational field. If it is pushed, the stone will roll down the hill, converting potential energy to kinetic energy.
We commonly refer to such other forms of energy as chemical energy and electrical energy. We can also understand these forms in terms of kinetic and potential energy; but in most of this book we shall be less specific about it and just say "energy." The form of energy with which we are most concerned in astronomy is radiant energy. The important point to remember throughout is that the conversion of energy from one form to another does not create energy or destroy energy.
