1. Units, Physical Quantities, and Vectors

A model is a simplified version of a physical system that is too complicated to analyze in full detail.

Any number that is used to describe a physical phenomenon quantitatively is a physical quantity. Some physical quantities are so fundamental that they can be defined only by describing how to measure them. Such a definition is called an operational definition. Three fundamental quantities are mass, length, and time.

A scalar quantity is a physical quantity described by a single number. A vector quantity has both a magnitude and a direction in space. Displacement (a vector quantity) is a change in the position of an object. If two vectors have the same direction, they are parallel. If two vectors have the same magnitude and the same direction, they are equal. The negative of a vector is a vector having the same magnitude as the original vector but the opposite direction. If two vectors have opposite directions, they are antiparallel.

—July 2020