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,
a,
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,
F.
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,
L = r × p.
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,
F,
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.
B
Basis vector
A vector of magnitude 1 along one of the
coordinate axes. Generally, we take the basis vectors to be
x
and
y,
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,
ß,
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.
C
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,
µk,
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,
µs
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,
x
and
y;
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
A × B = ABcos?n,
where
n
is a unit vector perpendicular to both A
and B. To deine which
direction
n
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.
D
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.
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.
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