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.