The term speed and velocity are often used interchangeably. In physical science, however, these terms have distinct meanings. The basic difference is that one is a scalar (speed) quantity and one is a vector (velocity). Let's generally distinguish between scalars and vectors now, because other terms will fall into these categories during the course of our study. The distinction is simple. A scalar quantity is one that has magnitude or size only (plus the unit of measurement). For example, you may be traveling in a car at 55 mi/h (about 90 km/h). This figure is your speed, which is a scalar quantity - magnitude (and units) only.

A vector quantity, on the other hand, is one that has magnitude and direction. For example, suppose you are traveling 55 mi/h north. This quantity describing your velocity, which is a vector quantity - magnitude (and units) plus direction. Because they give direction, also, vector quantity give more information than scalar quantities.

Now let's look more closely at speed and velocity as used in the description motion. Please choose one to continue: