Speed is considered an incomplete quantity to describe motion because it only tells us how fast an object is moving, but not in which direction.

Here's why:

* Speed is a scalar quantity: It has only magnitude (e.g., 10 m/s).

* Velocity is a vector quantity: It has both magnitude and direction (e.g., 10 m/s east).

Let's consider an example:

* Scenario 1: A car travels at 60 mph. This only tells us the speed of the car. We don't know if it's moving north, south, east, west, or any other direction.

* Scenario 2: A car travels at 60 mph north. This tells us both the speed (60 mph) and the direction (north).

Therefore, velocity provides a more complete description of motion than speed alone.

In many situations, knowing the direction is essential for understanding the movement of an object. For example, in navigation, we need to know both the speed and direction of a ship or aircraft.