Energy is the ability to do work or cause change. There are many forms of energy including kinetic energy, which is the energy of motion, and potential energy, which is stored energy due to an object's position or shape. When one object exerts a force on another, energy is transferred through work. In a closed system like a roller coaster, the total energy remains constant although it is converted between kinetic, potential, thermal, and sound energy due to friction and other forces.