Momentum is a physical quantity that describes the motion of an object. It is calculated by multiplying an object's mass by its velocity. The greater the mass or speed of the object, the more momentum it has. Momentum is a vector quantity, meaning it has both magnitude and direction. In a closed system, the total momentum remains constant, according to the law of conservation of momentum. This principle is often observed in collisions, such as those involving cars or billiard balls. When two objects collide, their combined momentum before and after the collision remains the same, assuming no external forces act on them.