
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
 The force involved in beta decay that changes a proton to a neutron and releases an electron and a neutrino.
 Weber
 The unit of magnetic flux, equal to one T · m^{2}.
 Weight
 The gravitational force exerted on a given mass.
 Weightlessness
 The experience of being in free fall. If you are in a satellite, elevator, or other freefalling object, then you have a weight of zero Newtons relative to that object.
 Work
 Done when energy is transferred by a force. The work done by a force F in displacing an object by s is W = F · s.
 Work function
 The amount of energy that metal must absorb before it can release a photoelectron from the metal.
 Workenergy theorem
 States that the net work done on an object is equal to the object’s change in kinetic energy.
Z
 Zeroth Law of Thermodynamics
 If two systems, A and B, are in thermal equilibrium and if B and C are also in thermal equilibrium, then systems A and C are necessarily in thermal equilibrium.
