Momentum is a vector quantity, meaning it has both magnitude (how much) and direction. An object can have negative momentum if its direction of motion is opposite to the chosen positive direction.

Here's a simple example:

* Imagine a car moving to the right. We define right as the positive direction. The car has positive momentum.

* Now, imagine another car moving to the left. Since left is the opposite of our chosen positive direction, this car has negative momentum.

Key points:

* Momentum is a vector: It's not just about speed, but also direction.

* The sign of momentum is determined by the direction. If the object is moving in the opposite direction of the positive direction, it has negative momentum.

* Negative momentum doesn't mean the object has less momentum. It just means it's moving in the opposite direction.

In physics, the choice of positive direction is arbitrary. You can choose any direction as positive, but it's important to be consistent throughout your problem.