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Science>Physics
Website, Terms Dictionary, A-Z Glossary of Definitions
Physics Dictionary - A-Z of Physics Terms with Definitions
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Glossary of Physics Terms
acceleration
rate of change in velocity with respect to time
(in units of distance/time2)
angular momentum
rotational analogue of momentum, in units of mass*length2/time
(see rotational kinematics)
Anthropic Principle
concept that states fundamentally that the Universe
is the way it is because if it were different we
would not exist to pose the question
black hole
a body whose escape velocity is greater than the
speed of light, causing gravity to pull back toward
the body any light it would otherwise emit (see
black holes)
center of charge
geometric point in the reference frame of a physical
body where that body's net charge can be said to
reside for physical purposes (as a "point charge")
center of mass
geometric point in the reference frame of a physical
body where that body's mass can be said to reside
for physical purposes (as a "point mass")
charge
fundamental property of matter (measured in Coulombs
in the metric system) which determines the electromagnetic
force an object exerts; macroscopically, excess
protons or electrons in an object
constant
any fundamental value (either with units or dimensionless)
inherent to the Universe; the fundamental physical
constants include the speed of light c, Planck's
constant h, the gravitational constant G, Boltzmann's
constant k, the gas constant R, and many others
current
the rate of moving charge (measured in Amperes in
the metric system)
dark matter
the "missing" mass of the Universe known to exist
because more mass than can be observed is needed
for a universe of ours' size and stability, possibly
in the form of small black holes, particles currently
thought to be massless, or mass exerting gravitational
forces fourth-dimensionally
Doppler effect
the result of frequencies of waves being observed
differently because of the observer's reference
frame
duality
the concept that electrons (and all particles) behave
both as classical particles and as waves depending
on how they are observed (so never both at the same
time)
energy
units of mass*length2/time2 (measured in Joules
in the metric system) that is conserved in any closed
system (see kinetic energy and potential energy)
entropy
the disorder of a system, said always to increase
with time by the second law of thermodynamics
escape velocity
the required speed of an object to escape another
body (such as a planet), reaching a velocity of
zero at infinity
event horizon
surface surrounding a black hole (singularity) outside
of which internal measurements except mass, electric
charge, and rotation velocity become immeasurable
and matter, normally, can no longer escape from
the black hole's gravitational field due to its
high escape velocity
force
units of mass*length/time2 (measured in Newton's
in the metric system) that determines the motion
of objects
frame of reference
distinct perspective of an event and its results
based on an observer's motion (see special relativity)
frequency
cycles per second (measured in units of time–1 measured
in Hertz in the metric system)
general relativity
theory that explains the relations of spacetime
and gravity (see relativity)
grand unified theory
a yet-unknown explanation of the Universe's basic
particles so that the four forces are expressed
as a single force, thought to be very simple and
self-explanatory in the Universe during Planck time
gravity
attractive force directly proportional to the product
of the masses of and inversely proportional to the
distance between two objects; infinite field that
determines curvature of spacetime; the only force
that has not been mathematically unified with others
heat
energy of the motion of particles in a body
heat death of the Universe
time at which no work will be able to be done because
all heat energy in the Universe will be evenly distributed
(so that no further increase in entropy is possible)
inertia
the tendency of matter to move uniformly (at a constant
velocity; in a straight line at a constant speed)
kinematics
the study of objects moving with uniform acceleration
kinetic energy
energy of a moving object, found nonrelativistially
as mv2/2
length contraction
relativistic distortion of observed length due to
differently accelerated reference frames
light
electromagnetic radiation within the frequency range
400–750 nanometers
mass
the property of matter (measured in kilograms in
the metric system) which determines its inertia
and the gravitational forces it exerts
matter
mass and energy; fundamental component of the Universe
moment of inertia
the rotational analogue of mass, in units of mass*length2
(see rotational kinematics)
momentum
units of mass*length/time attributed to a body that
is conserved in any collision
photon
a particle of light, having energy hf
physics
the study of properties of the Universe via measuring
experiments
Planck time
the first 10–43 seconds of the Universe when time
as humans recognize it did not exist because gravity,
which defines spacetime, had not yet split from
the other forces; instead, a time system that allowed
more than one object to occupy the same space was
in operation (see singularity)
potential energy
energy stored by an object (e.g. in a spring or
by gravity)
power
rate of change in work with respect to time (in
units of mass*length2/time3 measured in Watts in
the metric system)
quasar
"quasi-stellar radio source," possibly the most
mysterious (and distant) class of objects in the
Universe, identified by their immense production
of a wide range of electromagnetic radiation despite
their relatively small size
red shift
lowered frequency of light from a source receding
from the observation point (such as a star or galaxy
moving away from Earth) as a result of the Doppler
effect
rotational kinematics
the study of objects moving with uniform angular
acceleration in circles (see kinematics)
simultaneity
whether two events occur at the same time, said
by special relativity to depend upon the observational
reference frame
singularity
the zero-dimensional "point" at the center of a
black hole or other significant object (such as
the Universe at the Big Bang) at which all conceptions
of space and time "break down" and become incomprehensible,
defined by Hawking as a point at which spacetime
curvature is infinite
space
three-dimensional realm dependent on the presence
of matter in which the Universe partially exists
spacetime
the four-dimensional Universe suggested by general
relativity
special relativity
kinematic theory that explains the relations between
light and matter (see relativity)
speed
the (directionless) scalar value of the velocity
vector
speed of light
c, approximately 3*108
m/s; the maximum speed at which an object can travel
(see relativity)
statistical mechanics
the study of the motion of constituent particles
in a gas or other object and how they contribute
to the whole
supernova
the death of a star, usually a very violent process
thermodynamics
the study of heat, work, and entropy on a level
more macroscopic than statistical mechanics
time
the fourth dimension of spacetime that allows events
to occur linearly as humans recognize them, presumably
in the direction of increased entropy
time dilation
relativistic incongruence of two interpretations
of time due to differently accelerated reference
frames
timeline
series of events of a distinguished reality
torque
the rotational analogue of force, in units of mass*length2/time2
(see rotational kinematics)
units
the quality of a number or variable (i.e. what it
measures: mass, length, time, speed, energy, etc.)
universe
set of all events that have been, are, or will be
observable
velocity
rate of change in position with respect to time
(in units of distance/time)
wavelength
distance between two similar points on a repeating
wave
work
the energy exacted upon an object in displacing
it
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