What must be true about the direction of momentum in physics?

Momentum is a vector quantity, which means it has both magnitude and direction. This is fundamental to understanding how momentum operates in physical systems. The direction of momentum is crucial because it influences how bodies interact during collisions or when subjected to forces.

For instance, in a two-body collision, not only the magnitudes of the momenta matter but also their directions dictate the outcome of the interaction. If two objects collide with equal momenta but in opposite directions, their combined momentum would be zero, whereas if they collide in the same direction, their momenta will add up, resulting in a different outcome.

Therefore, the direction of momentum is not just an accessory feature; it fundamentally determines how momentum will influence the motion and interactions of objects in a given scenario. Understanding the directional aspect of momentum is key to analyzing and predicting the behavior of physical systems.