The roller coaster shown here is a perfect example of different types of energy. Energy is either potential, meaning that it is stored and not yet in use, or kinetic, meaning that it is energy in motion or in use. Common sources of energy include gravity, chemicals, temperature differences, and solar energy. Energy continually surrounds us, and can easily be converted from one form to another. The chemical energy of the food in your last meal, for example, has been transformed into a different form of chemical energy in your body, and may become kinetic energy at your soccer practice this afternoon. This roller coaster converts kinetic to potential energy, and vice-versa. You will learn about several forms of energy and how they change from one to another in this chapter.
Regardless of what events occur, in all closed systems, energy will be conserved. Most of what we think of as "energy" is in fact kinetic energy, or the energy of motion. Energy that is stored for later use is called potential energy. When two objects collide but conserve momentum and kinetic energy, we call these collisions elastic, and can calculate initial and final velocities using the conservation of momentum and conservation of kinetic energy equations.