* Momentum involves both magnitude and direction. Linear momentum is calculated as the product of an object's mass (a scalar) and its velocity (a vector). Velocity has both magnitude (speed) and direction.

* Vectors require both magnitude and direction for complete description. A scalar quantity only needs a magnitude (like temperature or mass).

* Momentum can be added vectorially. If two objects collide, their momenta can be added together as vectors to find the total momentum of the system.

In summary: Linear momentum is a vector because it has both magnitude (the product of mass and speed) and direction (the same direction as the velocity).