Final answer:
Knowing both the speed and direction of an object's motion refers to the object's velocity, which is a vector quantity that includes both magnitude and direction.
Step-by-step explanation:
When you know both the speed and direction of an object's motion, you are aware of the object's velocity. Speed is simply a scalar quantity that tells us how fast an object is moving without regard to its direction of movement. However, velocity is a vector quantity, which means it includes both the magnitude of the speed and the direction in which the object moves. For example, stating that an object is moving at 20 kilometers per hour does not tell us about its direction, and hence, it is not the velocity. However, stating that an object is moving 20 kilometers per hour due south gives us the object's velocity.
The concept of velocity is critical in physics because it allows us to describe not only how fast something is traveling but also the path it is taking. This becomes particularly important when looking at phenomena like acceleration, which is a change in velocity. For instance, even if an object is moving at a constant speed in uniform circular motion, if its direction is constantly changing, it is experiencing an acceleration.