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.
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Uniform circular motion
The motion of a body in a circular path with constant speed.
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Unit vector
A unit vector is a vector with length?1.
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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.
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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.
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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.
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Vertex
The center of a mirror or lens.
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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.
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W
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Wave
A system with many parts in periodic, or repetitive, motion. The oscillations in one part cause vibrations in nearby parts.
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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.
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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.
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Weak nuclear force
The force involved in beta decay that changes a proton to a neutron and releases an electron and a neutrino.
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Weber
The unit of magnetic flux, equal to one T · m2.
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Weight
The gravitational force exerted on a given mass.
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Weightlessness
The experience of being in free fall. If you are in a satellite, elevator, or other free-falling object, then you have a weight of zero Newtons relative to that object.
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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.
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Work function
The amount of energy that metal must absorb before it can release a photoelectron from the metal.
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Work-energy theorem
States that the net work done on an object is equal to the object’s change in kinetic energy.
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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.