A vector is a mathematical object possessing, and fully described by, a magnitude and a direction. It’s possible to talk about vectors simply in terms of numbers, but it’s often a lot easier to represent them graphically as arrows. The vector’s magnitude is equal to the length of the arrow, and its direction corresponds to where the arrow is pointing. Physicists commonly refer to the point of a vector as its tip and the base as its tail.

There are a number of ways to label vectors. You may have seen vectors labeled or A. This book will follow the convention you’ll find on SAT II Physics: vectors are written in boldface and vector magnitudes in plain script. For example, vector A has magnitude A.

Vectors vs. Scalars

In contrast to a vector quantity, a scalar quantity does not have a direction; it is fully described by just a magnitude. Examples of scalar quantities include the number of words in this sentence and the mass of the Hubble Space Telescope. Vector quantities you’ll likely come across quite frequently in physics include displacement, s; velocity, v; acceleration, a; force, F; momentum, p; electric field, E; and magnetic field, B.

When in doubt, ask yourself if a certain quantity comes with a direction. If it does, it’s a vector. If it doesn’t, it’s a scalar.