Uniform motion: motion at a constant speed. Speed: the distance traveled in a given time interval divided by the time interval. Velocity: the displacement divided by the time interval. Speed measures only how fast an object moves, but velocity tells us both an object"s speed and its direction. The (cid:498)per(cid:499) in meters per second unit of the denominator. The word per thus associates the number of units in the numerator with one. Chapter 1. 5: vectors and motion: a first look. Scalar quantity: a physical quantity described by a single number; can be positive, negative, or zero. Vector quantity: quantity that has both a size and a direction. Magnitude: the size or length of a vector; can be positive or zero but not negative. Drawing an arrow over the letter that represents the vector allows us to distinguish between vector and scalar.