SparkNotes: SAT Physics: A

A–D

Absolute zero: The lowest theoretical temperature a material can have, where the molecules that make up the material have no kinetic energy. Absolute zero is reached at 0 K or –273º C.

Acceleration: A vector quantity defined as the rate of change of the velocity vector with time.

Activity: In radioactive substances, the number of nuclei that decay per second. Activity, A, will be larger in large samples of radioactive material, since there will be more nuclei.

Alpha decay: A form of radioactive decay where a heavy element emits an alpha particle and some energy, thus transforming into a lighter, more stable, element.

Alpha particle: A particle, , which consists of two protons and two neutrons. It is identical to the nucleus of a helium atom and is ejected by heavy particles undergoing alpha decay.

Amplitude: In reference to oscillation, amplitude is the maximum displacement of the oscillator from its equilibrium position. Amplitude tells how far an oscillator is swinging back and forth. In periodic motion, amplitude is the maximum displacement in each cycle of a system in periodic motion. The precise definition of amplitude depends on the particular situation: in the case of a stretched string it would be measured in meters, whereas for sound waves it would be measured in units of pressure.

Angle of incidence: When a light ray strikes a surface, the angle of incidence is the angle between the incident ray and the normal.

Angle of reflection: The angle between a reflected ray and the normal.

Angle of refraction: The angle between a refracted ray and the line normal to the surface.

Angular acceleration: A vector quantity, , equal to the rate of change of the angular velocity vector with time. It is typically given in units of rad/s².

Angular displacement: The net change, , in a point’s angular position, . It is a scalar quantity.

Angular frequency: A frequency, f, defined as the number of revolutions a rigid body makes in a given time interval. It is a scalar quantity commonly denoted in units of Hertz (Hz) or s^–1.

Angular momentum: A vector quantity, L, that is the rotational analogue of linear momentum. For a single particle, the angular momentum is the cross product of the particle’s displacement from the axis of rotation and the particle’s linear momentum, . For a rigid body, the angular momentum is a product of the object’s moment of inertia, I, and its angular velocity, .

Angular period: The time, T, required for a rigid body to complete one revolution.

Angular position: The position, , of an object according to a co-ordinate system measured in s of the angle of the object from a certain origin axis. Conventionally, this origin axis is the positive x-axis.

Angular velocity: A vector quantity, , that reflects the change of angular displacement with time, and is typically given in units of rad/s. To find the direction of the angular velocity vector, take your right hand and curl your fingers along the particle or body’s direction of rotation. Your thumb then points in the direction of the body’s angular velocity.

Antinode: The points midway between nodes on a standing wave, where the oscillations are largest.

Atom: The building blocks of all matter, atoms are made up of a nucleus consisting of protons and neutrons, and a number of electrons that orbit the nucleus. An electrically neutral atom has as many protons as it has electrons.

Atomic number: A number, Z, associated with the number of protons in the nucleus of an atom. Every element can be defined in s of its atomic number, since every atom of a given element has the same number of protons.

Axis of rotation: The line that every particle in the rotating rigid body circles about.

Basis vector: A vector of magnitude 1 along one of the coordinate axes. Generally, we take the basis vectors to be and , the vectors of length 1 along the x- and y-axes, respectively.

Beats: When two waves of slightly different frequencies interfere with one another, they produce a “beating” interference pattern that alternates between constructive (in-phase) and destructive (out-of-phase). In the case of sound waves, this sort of interference makes a “wa-wa-wa” sound, and the frequency of the beats is equal to the difference in the frequencies of the two interfering waves.

Beta decay: A form of radioactive decay where a heavy element ejects a beta particle and a neutrino, becoming a lighter element in the process.

Beta particle: A particle, , identical to an electron. Beta particles are ejected from an atom in the process of beta decay.

Bohr atomic model: A model for the atom developed in 1913 by Niels Bohr. According to this model, the electrons orbiting a nucleus can only orbit at certain particular radii. Excited electrons may jump to a more distant radii and then return to their ground state, emitting a photon in the process.

Boiling point: The temperature at which a material will change phase from liquid to gas or gas to liquid.

Boyle’s Law: For a gas held at a constant temperature, pressure and volume are inversely proportional.

Calorie: The amount of heat needed to raise the temperature of one gram of water by one degree Celsius. 1 cal = 4.19 J.

Celsius: A scale for measuring temperature, defined such that water freezes at 0ºC and boils at 100ºC. 0ºC = 273 K.

Center of curvature: With spherical mirrors, the center of the sphere of which the mirror is a part. All of the normals pass through it.

Center of mass: Given the trajectory of an object or system, the center of mass is the point that has the same acceleration as the object or system as a whole would have if its mass were concentrated at that point. In terms of force, the center of mass is the point at which a given net force acting on a system will produce the same acceleration as if the system’s mass were concentrated at that point.

Centripetal acceleration: The acceleration of a body experiencing uniform circular motion. This acceleration is always directed toward the center of the circle.

Centripetal force: The force necessary to maintain a body in uniform circular motion. This force is always directed radially toward the center of the circle.

Chain reaction: The particles and energy released by the fission or fusion of one atom may trigger the fission or fusion of further atoms. In a chain reaction, fission or fusion is rapidly transferred to a large number of atoms, releasing tremendous amounts of energy.

Charles’s Law: For a gas held at constant pressure, temperature and volume are directly proportional.

Coefficient of kinetic friction: The coefficient of kinetic friction, , for two materials is the constant of proportionality between the normal force and the force of kinetic friction. It is always a number between zero and one.

Coefficient of linear expansion: A coefficient that tells how much a material will expand or contract lengthwise when it is heated or cooled.

Coefficient of static friction: The coefficient of static friction, for two materials is the constant of proportionality between the normal force and the maximum force of static friction. It is always a number between zero and one.

Coefficient of volume expansion: A coefficient that tells how much the volume of a solid will change when it is heated or cooled.

Coherent light: Light such that all of the associated waves have the same wavelength and are in phase.

Collision: When objects collide, each object feels a force for a short amount of time. This force imparts an impulse, or changes the momentum of each of the colliding objects. The momentum of a system is conserved in all kinds of collisions. Kinetic energy is conserved in elastic collisions, but not in inelastic collisions. In a perfectly inelastic collision, the colliding objects stick together after they collide.

Completely inelastic collision: A collision in which the colliding particles stick together.

Component: Any vector can be expressed as the sum of two mutually perpendicular component vectors. Usually, but not always, these components are multiples of the basis vectors, and ; that is, vectors along the x-axis and y-axis. We define these two vectors as the x- and y-components of the vector.

Compression: An area of high air pressure that acts as the wave crest for sound waves. The spacing between successive compressions is the wavelength of sound, and the number of successive areas of compression that arrive at the ear per second is the frequency, or pitch, of the sound.

Concave lens: Also called a diverging lens, a lens that is thinner in the middle than at the edges. Concave lenses refract light away from a focal point.

Concave mirror: A mirror that is curved such that its center is farther from the viewer than the edges, such as the front of a spoon. Concave mirrors reflect light through a focal point.

Conduction: Heat transfer by molecular collisions.

Conservation of Angular Momentum: If the net torque acting on a rigid body is zero, then the angular momentum of the body is constant or conserved.

Conservation of momentum: The principle stating that for any isolated system, linear momentum is constant with time.

Constant of proportionality: A constant in the numerator of a formula.

Constructive interference: The amplification of one wave by another, identical wave of the same sign. Two constructively interfering waves are said to be “in phase.”

Convection: Heat transfer via the mass movement of molecules.

Convex lens: Also called a converging lens, a lens that is thicker in the middle than at the edges. Convex lenses refract light through a focal point.

Convex mirror: A mirror that is curved such that its center is closer to the viewer than the edges, such as a doorknob. Convex mirrors reflect light away from a focal point.

Cosine: The cosine of an angle in a right triangle is equal to the length of the side adjacent to the angle divided by the length of the hypotenuse.

Crest: The points of maximum displacement along a wave. In traveling waves, the crests move in the direction of propagation of the wave. The crests of standing waves, also called anti-nodes, remain in one place.

Critical angle: For two given media, the smallest angle of incidence at which total internal reflection occurs.

Cross product: A form of vector multiplication, where two vectors are multiplied to produce a third vector. The cross product of two vectors, A and B, separated by an angle, , is , where is a unit vector perpendicular to both A and B. To deine which direction points, you must use the right-hand rule.

Cycle: In oscillation, a cycle occurs when an object undergoing oscillatory motion completes a “round-trip.” For instance, a pendulum bob released at angle has completed one cycle when it swings to and then back to again. In period motion, a cycle is the sequence through which a system once during each oscil-lation. A cycle can consist of one trip up and down for a piece of stretched string, or of a compression followed by a rarefaction of air pressure for sound waves.

De Broglie wavelength: A wavelength, given by = h/mv, which is associated with matter. Louis de Broglie proposed the idea that matter could be treated as waves in 1923 and applied this theory successfully to small particles like electrons.

Decay constant: A constant, , not to be confused with wavelength, that defines the speed at which a radioactive element undergoes decay. The greater is, the faster the element decays.

Decibel: A logorithmic unit for measuring the volume of sound, which is the square of the amplitude of sound waves.

Deposition: The process by which a gas turns directly into a solid because it cannot exist as a liquid at certain pressures.

Destructive interference: The cancellation of one wave by another wave that is exactly out of phase with the first. Despite the dramatic name of this phenomenon, nothing is “destroyed” by this interference—the two waves emerge intact once they have passed each other.

Diffraction: The bending of light at the corners of objects or as it passes through narrow slits or apertures.

Diffraction grating: A sheet, film, or screen with a pattern of equally spaced slits. Typically the width of the slits and space between them is chosen to generate a particular diffraction pattern.

Direction: The property of a vector that distinguishes it from a scalar: while scalars have only a magnitude, vectors have both a magnitude and a direction. When graphing vectors in the xy-coordinate space, direction is usually given by the angle measured counterclockwise from the x-axis to the vector.

Directly proportional: Two quantities are directly proportional if an increase in one results in a proportional increase in the other, and a decrease in one results in a proportional decrease in the other. In a formula defining a certain quantity, those quantities to which it's directly proportional will appear in the numerator.

Dispersion: The separation of different color light via refraction.

Displacement: A vector quantity, commonly denoted by the vector s, which reflects an object’s change in spatial position. The displacement vector points from the object’s starting position to the object’s current position in space. If an object is moved from point A to point B in space along path AB, the magnitude of the object’s displacement is the separation of points A and B. Note that the path an object takes to get from point A to point B does not figure when deining displacement.

Distance: A scalar quantity. If an object is moved from point A to point B in space along path AB, the distance that the object has traveled is the length of the path AB. Distance is to be contrasted with displacement, which is simply a measure of the distance between points A and B, and doesn’t take into account the path followed between A and B.

Doppler shift: Waves produced by a source that is moving with respect to the observer will seem to have a higher frequency and smaller wavelength if the motion is towards the observer, and a lower frequency and longer wavelength if the motion is away from the observer. The speed of the waves is independent of the motion of the source.

Dot product: A form of vector multiplication, where two vectors are multiplied to produce a scalar. The dot product of two vectors, A and B, is expressed by the equation A · B = AB cos .

Dynamics: The application of kinematics to understand why objects move the way they do. More precisely, dynamics is the study of how forces cause motion.