The pitcher winds up, then pitches. He demonstrates both potential energy in the windup, and kinetic energy in the pitch. Potential energy is stored energy ready to release: a roller coaster at the top of its first peak, a car ready to descend a San Francisco street, an eager student ready to leave his desk. The subsequent action is kinetic energy -- the energy of motion released. Both apply to numerous everyday situations.

The electricity that fuels people's homes is supplied by potential energy turned kinetic, either in the form of an electric plant fueled by coal, a hydroelectric dam, or other source such as solar cells. The coal is stored potential energy at its most inert; it must be burned to translate itself into kinetic energy. The water behind the dam is, despite its eddies and currents, relatively inert as well, but it also supplies power when it is transformed by flowing through the dam and transferring it kinetic energy. Switch on the light. The switch's movement releases potential energy, while the light is kinetic.

Cars on the road offer another example of potential-to-kinetic energy, whether driving a gasoline-fueled automobile or an electrically powered model. The fuel stored in a gasoline-powered car's tank is potential energy, ready to be used for transportation; the ignition, spark and firing of the engine begin the potential-to-kinetic cycle, and the car's response as it leaves the driveway and heads onto the road is an extension of kinetic movement. Electric cars store their potential energy in batteries, waiting for the switch-on that begins their driver's kinetically-powered trip.

Kinetic energy seldom ends with a single reaction. In sports, for example, the release of potential energy found in a tautly-strung tennis racket or a drawn bow -- called elastic potential energy -- often results in several kinetic reactions. When you hit a tennis ball, the kinetic energy is released in the ball's flight, but it redoubles in energy and speed if your opponent returns the ball to you. The stored potential energy of the strung racket is transferred to the ball's kinetic explosion of flight.

Rainwater becomes a dam's power resource. A moving car hits a stationary car, causing it to move as well. A football sails towards a quarterback, while a baseball crashes through a window. All these potential-to-kinetic actions and reactions are examples of the law of conservation of energy, which reminds us that energy is never destroyed, but only transferred, moving from the rainy sky to the rushing dam, or from the baseball player's hand to the shattered window.