## Potential energy

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# POTENTIAL ENERGY

Potential energy is the energy that something has because of its position or because of the arrangement of its parts. A baseball in flight has potential energy because of its position above the ground. A carbohydrate molecule has potential energy because of the arrangement of the atoms in the molecule.

A diver standing on a platform above the water has potential energy because of a capacity for doing work by jumping off. The higher the diver is above the water and the greater the diver's mass, the greater the potential energy—that is, the capacity for doing work. It is the gravitational force that pulls the diver downward and into the water. For this reason, the potential energy of the diver is called "gravitational potential energy." As an equation, gravitational potential energy (U) measured in joules (J) is given by U = mgh, where mis the mass in kilograms (kg), g is the acceleration due to the gravitational force (9.80 meters/second2, and h is the height in meters (m). An 80-kg diver 3 m above the water would have 2,350 J of gravitational potential energy.

Any type of potential energy is associated with a force and involves some favorable position. A compressed spring in a toy gun does work on a projectile when the spring is released. Before release, the spring had potential energy that we label "elastic potential energy." The force involved is that of the spring, and the favorable position is the amount of compression. Other examples of elastic potential energy are involved in flexing the bow of a bow and arrow, flexing a vaulter's pole, compressing the suspension springs of an automobile, and stretching a rubber band.

A loss in potential energy by a system is accompanied by a gain of energy in some other form. All the potential energy of a mass held above a spring is converted to kinetic energy just before the mass hits the spring. The mass loses kinetic energy as it pushes against the spring, but the spring gains potential energy as a result of being compressed. Water atop a dam in a hydroelectric power plant has gravitational potential energy. In falling, it loses potential energy and gains kinetic energy. When the water impinges on the blades of a water turbine, the rotor of the turbine gains rotational kinetic energy at the expense of the water losing kinetic energy.

Molecules have potential energy associated with electric forces that bind the atoms together. In a chemical reaction liberating energy—heat, for example—the atoms are rearranged into lower potential energy configurations. The loss of potential energy is accompanied by an increase in other forms of energy.

Protons and neutrons in the nucleus of an atom have potential energy associated with nuclear forces. In a nuclear reaction liberating energy, the nuclei are rearranged into lower potential energy structures. The loss of potential energy is accompanied by an increase in other forms of energy. A single nuclear reaction in a nuclear power plant liberates nearly 10 million times the energy of a single chemical reaction in a coal-burning power plant.

Joseph Priest

## BIBLIOGRAPHY

Hobson, A. (1995). Physics: Concepts and Connections. Englewood Cliffs, NJ: Prentice-Hall.

Priest, J. (2000). Energy: Principles, Problems, Alternatives, 5th ed. Dubuque, IA: Kendall/Hunt Publishing Co.

Serway, R. A. (1998). Principles of Physics, 2nd ed. Forth Worth, TX: Saunders College Publishing.

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potential energy Type of energy an object possesses because of its vertical position in the Earth's gravitational field; also the energy stored in a system such as a compressed spring or in an oscillating system such as a pendulum. An object on a shelf has potential energy given by mgh, where m is its mass, g the acceleration due to gravity, and h the height of the shelf.

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potential energy See ELEVATION POTENTIAL ENERGY; and HYDRAULIC HEAD.