A - The symbol for ampere. Also when in lower-case italic, the symbol for acceleration (a).

Absolute zero - The temperature at which a substance has no kinetic energy to give up. This temperature corresponds to 0 K, or to -273ºC.

Acceleration - The rate at which velocity is changing. The change may be in magnitude, direction, or both.

Air Resistance - The friction that acts on something moving through air.

Alternating Current (ac) - Electric current that rapidly reverses in direction, usually at the rate of 60 hertz (in North America) or 50 hertz (in most other places).

Ampere - The SI unit of electric current. One ampere (symbol A) is equal to a flow of one coulomb of charge per second.

Amplitude - The distance from the midpoint to the crest of a wave or, equivalently, from the midpoint to the trough.

Angle of Incidence - The angle between an incident ray and the normal to a surface.

Angle of Reflection - The angle between a reflected ray and the normal to a surface.

Angular Momentum - The "inertia of rotation" of a rotating object, equal to the product of rotational inertia and rotational velocity.

Apogee - The point in an elliptical orbit where an object is farthest away from the object about which it orbits.

Apparent Weightlessness - The feeling of weightlessness that one has when falling toward or around the earth (as in an orbiting spacecraft). True weightlessness, however, requires that an object be far out in space, where gravitational forces are negligible.

Archimedes' Principle - The relationship between buoyancy and displaced fluid: an immersed object is buoyed up by a force equal to the weight of the fluid it displaces.

Atom - The smallest particle of an element that can be identified with that element. It consists of protons and neutrons in a nucleus surrounded by electrons.

Atomic Mass Number - The total number of nucleons (neutrons and protons) in the nucleus of an atom.

Atomic Number - The number of protons in the nucleus of an atom.

Average Speed - The total distance covered divided by the time and interval.

Axis - The straight line about which rotation takes place.

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Beats - A throbbing variation in the loudness of a sound caused by interference when two tones of slightly different frequencies are sounded together.

Bernoulli's Principle - The statement that the pressure in a fluid decreases as the speed of the fluid increases.

Black Hole - A massive star that has collapsed to so great a density that its enormous local gravitational field prevents light from escaping and it thus appears black.

Blue Shift - An increase in the measured frequency of light from an approaching source; called the blue shift because the increase is toward the high-frequency, or blue, end of the spectrum.

Boiling - The change of state from liquid to gas that occurs beneath the surface of the liquid. The gas that forms beneath the surface occurs as bubbles, which rise to the surface and escape.

Boyle's Law - The statement that the product of pressure and volume for a given mass of gas is a constant as long as the temperature does not change.

Breeder Reactor - A nuclear fission reactor that not only produces more nuclear fuel than it consumes by converting a nonfissionable uranium isotope into a fissionable plutonium.

Brownian Motion - The random movement observed among microscopic particles suspended in a fluid medium.

Buoyancy - The apparent loss of weight of an object submerged in a fluid.

Buoyant Force - The net upward force exerted by a fluid on a submerged object.

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- The symbol for coulomb. Also, when preceded by the degree symbol °, the symbol for Celsius.

cal - The symbol for calorie.

Calorie - A unit of heat. One calorie (symbol cal) is the amount of heat required to raise the temperature of one gram of water by 1 degree Celsius. One Calorie (with a capital C) is equal to one thousand calories and is the unit used in describing the energy available from food.

Celsius Scale - A temperature scale in which the number 0 is assigned to the temperature at which water freezes, and the number 100 is assigned to the temperature at which water boils (at standard pressure).

Center of Gravity - The point at the center of an object's weight distribution, where the force of gravity can be considered to act.

Center of Mass - The point at the center of an object's mass distribution, where all its mass can be considered to be concentrated. For everyday conditions, it is the same as the center of gravity.

Centrifugal Force - An apparent outward force experienced by a rotating body. It is fictitious in the sense that it is not part of an interaction but is due to the tendency of a a moving body to follow a straight-line path.

Centripetal Force - A center-seeking force that causes an object to follow a circular path.

Chain Reaction - A self-sustaining reaction that, once started, steadily provides the energy and matter necessary to continue the reaction.

Charge - The property to which is attributed the mutual repulsion of two electrons or two protons, and the mutual attraction of an electron and a proton. Also, the sum of all the electron and proton charges on an object (allowing for the cancelling effect of equal numbers of like and unlike charges).

Circuit - Any complete path along which charge can flow.

Coherent - Type of light beam in which the waves all have the same frequency, phase, and direction. Lasers produce coherent light.

Complementary Colors - Two colors of light beams which when added together appear white.

Component - One of the vectors in different directions whose vector sum is equal to a given vector. Any single vector may be regarded as the vector sum of two components, each of which acts in a different direction.

Compound - A chemical substance made of atoms of two or more different elements combined in a fixed proportion.

Condensation - (a) The change of a state of a gas into a liquid; the opposite of evaporation. (b) In sound, a pulse of compressed air (or other matter).

Conduction - A means of heat transfer within certain materials and from one material to another when the two are in direct contact. It involves the transfer of energy from atom to atom.

Conductor - (a) A material through which heat can flow. (b) A material, usually a metal, through which electric charge can flow. Good conductors of heat are generally good conductors of charge.

Conservation of Charge - The principle that net electric charge is neither created nor destroyed but simply transferred from one material to another.

Conserved - Term applied to a physical quantity, such as momentum, energy, or electric charge, that remains unchanged after some interaction.

Constructive Interference - Addition of two waves when the crest of one wave overlaps the crest of another, so that their individual effects add together. The result is a wave of increased amplitude

Convection - A means of heat transfer by movement of the heated substance itself, such as by currents in a fluid.

Converging Lens - A lens that is thickest in the middle and that causes parallel rays of light to converge to a focus.

Correspondence Principle - The principle that for a new theory to be valid, it must account for the verified results of the old theory in the region where both theories are applicable.

Coulomb - The SI unit of charge. One coulomb (symbol C) is equal to the total charge of 6.25x10^18.

Coulomb's Law - The relationship among electrical force, charges, and distance: The electrical force between two charges varies directly as the product of the charges and inversely as the square of the distance between them.

Crest - One of the places in a wave where the wave is the highest or the disturbance is greatest.

Critical Angle - The minimum angle of incidence at which a light ray is totally reflected within a medium.

Critical Mass - The minimum mass of fissionable material in a nuclear reactor or nuclear bomb that will sustain a chain reaction.

Current - See electric current.

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Density - A property of a substance, equal to the mass divided by the volume; commonly thought of as the "lightness" or "heaviness" of a substance.

Destructive Interference - Addition of two waves when the crest of one wave overlaps the trough of another, so that their individual effects cancel each other. The result is a wave of decreased amplitude.

Diffraction - The bending of a wave around a barrier, such as an obstacle or the edges of an opening.

Diffraction Grating - A series of closely-spaced parallel slits which are used to separate colors of light by interference.

Displaced - Term applied to the fluid that is moved out of the way when an object is placed in the fluid. A completely submerged object always displaces a volume of fluid equal to its own volume.

Diverging Lens - lens that is thinnest in the middle and that causes parallel rays of light to diverge.

Doppler Effect - The apparent change is frequency of a wave due to the motion of the source or the receiver.