Ultraviolet – the region of the electromagnetic spectrum between visible light and x rays

Uncertainty principle – there is a limit to how accurately simultaneous measurements of position and momentum (or time and energy) can be made (see equation 28.6)

Unified theory – a comprehensive theory that relates the four basic forces in nature

Uniform circular motion – the motion of a particle moving along a circular path with a constant speed

Unit cell – the smallest unit of a crystal that has the repeated structure of the crystal

Universal gas constant – the proportionality constant in the ideal gas law

Universal gravitational constant - the proportionality constant in the law of universal gravitation