NASA - National Aeronautics and Space Administration

Energy is the ability to do work. Most of the energy on Earth comes, in one way or another, from the sun. It travels to the Earth in the sun's rays. Plants use the sun's rays to make food. Animals and human beings eat plants and other animals and use the energy in this food to move and live. All living things need energy to stay alive.

People have always looked for sources of energy to do their work. The earliest people had only the strength of their bodies. They later discovered how to make sails that used the energy of the wind to move boats. People also learned how to use the energy of water to turn the mills that ground their grain. They tamed animals and used their energy to pull plows and wagons.

Image to right: Swinging on a swing shows potential, or stored, energy and kinetic, or moving, energy. Credit: World Book diagram

People have now found many ways to release energy to do work. They burn coal to turn water into steam. Then they use the steam to generate electrical energy. They turn this electrical energy into radio waves that carry information and ideas for thousands of miles. People release the energy in gasoline by burning it to power automobiles. They build power plants that use certain kinds of atoms to release enormous amounts of energy.

Energy has a potential (stored up) form and a kinetic (moving) form, and it changes from one form to the other. A girl who swings backward on a swing set has stored-up, or potential, energy at the top of her swing. After she swings down from the top, kinetic energy, or the energy of movement, swings her forward.

How to cite this article: To cite this article, World Book recommends the following format: "Energy." The World Book Student Discovery Encyclopedia. Chicago: World Book, Inc., 2005.

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