We live in a 3-dimensional world; most significant cases of motion necessarily involve at least 2 and often all 3 dimensions. The richness of multiple dimensions requires a new mathematical tool for quantifying motion: the vector. A vector is a quantity that has both size—its magnitude—and direction. As in 1 dimension, the description of motion with constant acceleration in multiple dimensions is straightforward. Near Earth’s surface, the result is so-called projectile motion, which provides a good approximation to the motions of objects.