* Velocity is a vector: Velocity has both magnitude (speed) and direction. A negative velocity simply indicates a direction opposite to a chosen positive direction.
* Momentum inherits the direction of velocity: Because momentum is directly proportional to velocity, it also has a direction.
* Negative momentum means movement in the opposite direction: If an object is moving in the negative direction (like a car moving backward), its velocity will be negative, and therefore its momentum will also be negative.
In short: A negative momentum simply means the object is moving in the opposite direction to whatever you've defined as the positive direction.
