When you know both the speed and direction of a motion, you have a complete description of the velocity of that motion.

Here's why:

* Speed tells you how fast something is moving. It's a scalar quantity, meaning it only has magnitude (e.g., 50 mph).

* Direction tells you which way the motion is heading. It's a vector quantity.

* Velocity combines both speed and direction, making it a vector quantity as well.

For example:

* Speed: A car is traveling at 60 mph.

* Direction: The car is traveling north.

* Velocity: The car is traveling at 60 mph north.

Knowing the velocity gives you a more complete understanding of the motion than just knowing the speed alone.