Glossary
A
- abrasion
a form of mechanical weathering that occurs when rocks scrape or grind against one another (p. 710)
- absolute age
the time that has passed since a rock formed (p. 734)
- absolute brightness
a description of how bright a star really is; a characteristic property of a star that does not depend on the star's distance from Earth (p. 837)
- absolute zero
- absorption lines
a set of dark lines that show frequencies at which light has been absorbed from a star's bright spectrum (p. 837)
- acceleration
the rate at which velocity changes (p. 342)
- accretion
the process of adding mass in the formation of the planets by collisions between planetesimals (p. 819)
- accuracy
the closeness of a measurement to the true value of what is measured (p. 19)
- acid
a compound that produces hydronium ions (H3O+) when dissolved in water; a proton donor (p. 241)
- actual mechanical advantage
the ratio of the output force to the input force in a machine (p. 422)
- air mass
a large body of air that has consistent properties throughout, such as temperature and moisture content, at a given altitude (p. 765)
- air pressure
the pressure caused by the weight of a column of air in Earth's atmosphere (p. 748)
- air resistance
fluid friction acting on an object moving through the air (p. 360)
- alkali metals
the elements in Group 1A of the periodic table, not including hydrogen (p. 140)
- alkaline earth metals
the elements in Group 2A of the periodic table (p. 141)
- alloy
a mixture of two or more elements, at least one of which is a metal, that has the characteristic properties of a metal (p. 178)
- alluvial fan
a fan-shaped deposit of sediment, on land formed as a stream flows out of the mountains and onto a plain (p. 716)
- alpha particle
a positively charged particle, emitted by certain radioactive nuclei, made up of two protons and two neutrons; a helium nucleus (p. 293)
- alternating current (AC)
a flow of electric charge that regularly reverses its direction (p. 604)
- amino acids
compounds that contain both carboxyl and amino functional groups (p. 280)
- amplitude
the maximum displacement of a medium from the rest position (p. 507)
- amplitude modulation (AM)
a method of transmitting a radio signal in which the amplitude of the carrier wave varies while its frequency remains the same (p. 541)
- analog signal
a smoothly varying signal produced by continuously changing the voltage or current in a circuit (p. 619)
- angle of incidence
the angle an incident ray makes with a line perpendicular to a surface it strikes (p. 570)
- angle of reflection
the angle a reflected ray makes with a line perpendicular to a surface it strikes (p. 570)
- angle of refraction
the angle a light ray makes with the normal after it enters a new medium at an angle (p. 575)
- anion
an ion with a negative charge (p. 160)
- anticyclone
a weather system with a swirling center of high air pressure (p. 769)
- antinode
a point of maximum displacement midway between two nodes in a standing wave (p. 512)
- apparent brightness
the brightness of a star as it appears from Earth (p. 836)
- aquifer
a permeable rock layer that is saturated with water (p. 707)
- Archimedes' principle
the equivalence of the buoyant force on an object and the weight of the fluid displaced by the object (p. 401)
- aromatic hydrocarbons
hydrocarbons that contain a ring structure similar to benzene (p. 266)
- asteroid belt
region between the orbits of Mars and Jupiter where most asteroids in the solar system are found (p. 809)
- asteroids
small, rocky solar-system bodies, most of which are found orbiting the sun in a region between Mars and Jupiter (p. 809)
- asthenosphere
a layer of softer, weaker rock beneath Earth's lithosphere, which can flow slowly (p. 662)
- astigmatism
an eye condition in which objects at any distance appear blurry because of the distorted shape of the cornea (p. 592)
- astronomical unit (AU)
a unit of distance that equals the average distance from Earth to the sun; 149,598,000 kilometers (p. 793)
- astronomy
the study of the universe beyond Earth (p. 4)
- atmosphere
the layer of gases that surrounds Earth (p. 747)
- atom
the smallest particle of an element (p. 39)
- atomic mass unit (amu)
one twelfth the mass of a carbon-12 atom (p. 134)
- atomic number
a unique number for each element that equals the number of protons in an atom of that element (p. 110)
- aurora
a colorful display of light in the ionosphere, produced when charged particles from the sun are attracted to Earth's magnetic poles (p. 751)
- average speed
the total distance traveled divided by the time it takes to travel that distance (p. 333)
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