Potential energy is the energy，chemical or physical，stored within an object，atom or molecule（分子）．Think about a car at the top of a roller-coaster （過(guò)山車），pausing just before it slopes into the next turn．A log resting in a fireplace just before it is about to be lit is a treasure house of potential energy．As the log burns，the connections between carbon atoms that make up the wood are being broken down，and the potential energy stored within those connections is being released as heat and light．As a comet approaches a planet or star，it slows，momentarily affected by the larger body's gravity．The potential energy builds up and then reaches a breaking point as the comet accelerates around the larger body，and is shot out to the other side．
    Sports show countless examples every day of potential energy being transformed into kinetic energy．Kinetic energy is the energy of movement．When an archer（弓箭手）draws and holds her arrow，her bow is filled with stored potential energy．When she releases the bowstring，all the potential energy is quickly transformed into kinetic energy，which is transferred to the arrow that takes flight．
    The transformation of stored potential energy into kinetic energy can also be made use of to power homes，factories and entire cities．The most notable example is the Hoover Dam．The Hoover Dam is an arch-gravity dam by design．This design name is the first clue as to how exactly the dam makes use of energy．Located in the Black Canyon of the Colorado River，the Hoover Dam formed，and now holds back Lake Mead - the largest reservoir in the United States．
    Gravity acts as a force upon Lake Mead．Held at bay by the Hoover Dam，the waters of Lake Mead and the Colorado River gain greater potential energy with each passing moment．The Arizona and Nevada spillways（溢洪道）are two means by which the waters of Lake Mead can escape the dam．As the lake water falls over the walls into a spillway，potential energy is instantly transformed into kinetic energy．The channels through which the water normally escapes every day are the four intake towers．These towers allow the water flow through passageways to the powerhouse and hydroelectric generators．When the water reaches the intake towers and is allowed to flow down through the passageways，all the stored potential energy created by the force of gravity acting upon the water is transformed into kinetic energy，just as when water flows over the wall into a spillway．By taking advantage of the transformed potential energy of Lake Mead，the Hoover Dam provides power to California，Nevada and Arizona．
    Moments like these happen all across the physical world，whether on the molecular or universe level．

Main points	Supporting details
What is potential energy？	Potential energy is the energy existing either in the chemical or physical （1） form/state form/state ，which is stored in an object． ? When a car is sloping into the next turn from the top of a roller-coaster，it is building up potential energy． ? When a log is burning，the connections between carbon atoms are being broken down，（2） releasing releasing potential energy as light and heat． ? When a comet approaches a planet or star，the potential energy （3） accumulates accumulates to a breaking point and then the comet is shot out to the other side．
How is potential energy transformed into kinetic energy？	Potential energy can be transformed into kinetic energy，which is the energy （4） produced / generated / created． produced / generated / created． by movement． ? When the bowstring is released by an archer，the potential energy stored in the full bow is quickly transferred to the arrow，thus transformed into kinetic energy which sets the arrow （5） flying flying far away． ? Hoover Dam is the most notable example of energy transformation． ? Hoover Dam is （6） located/situated located/situated in the Black Canyon of the Colorado River，holding back Lake Mead． ? The waters of Lake Mead escape the Dam，falling over the walls into Arizona and Nevada spillways，leading to the （7） instant instant transformation from potential energy into kinetic energy． ? The （8） creation / formation creation / formation of potential energy is from the force of gravity acting upon the water
What （9） use use is the energy transformation made of？	Homes，factories and cities are efficiently （10） powered powered thanks to the powerhouses and hydroelectric generators taking advantage of the transformed potential energy．