Traveling waves
A wave with wave crests that propagate down the length of the medium, in contrast to
stationary standing waves. The velocity at which a crest propagates is called the wave speed.
Trough
The points of maximum negative displacement along a wave. They are the opposite of wave
crests.
U–Z
U
Uncertainty principle
A principle derived by Werner Heisenberg in 1927 that tells us that we can never know both the
position and the momentum of a particle at any given time.
Uniform circular motion
The motion of a body in a circular path with constant speed.
Unit vector
A unit vector is a vector with length 1.
Universal gas constant
Represented by R = 8.31 J/mol · K, the universal gas constant fits into the ideal gas law so as to
relate temperature to the average kinetic energy of gas molecules.
V
Vector
A vector quantity, or vector, is an object possessing, and fully described by, a magnitude and a
direction. Graphically a vector is depicted as an arrow with its magnitude given by the length of
the arrow and its direction given by where the arrow is pointing.
Velocity
A vector quantity defined as the rate of change of the displacement vector with time. It is to be
contrasted with speed, which is a scalar quantity for which no direction is specified.
Vertex
The center of a mirror or lens.
Virtual image
An image created by a mirror or lens in such a way that light does not actually come from where
the image appears to be.
W
Wave
A system with many parts in periodic, or repetitive, motion. The oscillations in one part cause
vibrations in nearby parts.
Wave speed
The speed at which a wave crest or trough propagates. Note that this is not the speed at which
the actual medium (like the stretched string or the air particles) moves.
Wavelength
The distance between successive wave crests, or troughs. Wavelength is measured in meters
and is related to frequency and wave speed by = v/f.
Weak nuclear force