Energy - Key terms


A law of physics which holds that within a system isolated from all other outside factors, the total amount of energy remains the same, though transformations of energy from one form to another take place.


For an acute (less than 90°) in a right triangle, the cosine (abbreviated cos) is the ratio between the adjacent legand the hypotenuse. Regardless of the size of the triangle, this figure is a constant for any particular angle.


The ability of an object (or in some cases a non-object, such as a magnetic force field) to accomplish work.


The force that resists motion when the surface of one object comes into contact with the surface of another.


The British unit of power, equal to 550 foot-pounds per second.


In a right triangle, the side opposite the right angle.


The SI measure of work. One joule (1 J) is equal to the work required to accelerate 1 kilogram of mass by 1 meter per second squared (1 m/s 2 ) over a distance of 1 meter. Due to the small size of the joule, however, it is often replaced by the kilowatt-hour, equal to 3.6 million(3.6 · 106) J.


The energy that an object possesses by virtue of its motion.


Physical substance that occupies space, has mass, is composed of atoms (or in the case of subatomic particles, is part of an atom), and is convertible into energy.


The sum of potential energy and kinetic energy within a system.


The energy that an object possesses by virtue of its position.


The rate at which work is accomplished over time, a figure rendered mathematically as work divided by time. The SI unit of power is the watt, while the British unit is the foot-pound per second. The latter, because it is small, is usually reckoned in terms of horsepower.


The energy an object possesses by virtue of its mass.


A triangle that includes a right (90°) angle. The other two angles are, by definition, acute or less than 90°.


A quantity that possesses only magnitude, with no specific direction.


An abbreviation of the French Système International d'Unités, which means "International System of Units." This is the term within the scientific community for the entire metric system, as applied to a wide variety of quantities ranging from length, weight and volume to work and power, as well as electromagnetic units.


In discussions of energy, the term "system" refers to a closed set of interactions free from interference by outside factors. An example is the baseball dropped from a height to illustrate potential energy and kinetic energy the ball, the space through which it falls, and the ground below together form a system.


A quantity that possesses both magnitude and direction.


The metric unit of power, equal to 1 joule per second. Because this is such a small unit, scientists and engineers typically speak in terms of kilowatts, or units of 1,000 watts.


The exertion of force over a given distance. Work is the product of force and distance, where force and distance are exerted in the same direction. Hence the actual formula for work is F · cos θ · s, where F = force, s = distance, and cos θ is equal to the cosine of the angle θ (the Greek letter theta) between F and s. In the metric or SI system, work is measured by the joule (J), and in the British system by the foot-pound.

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