Which statement best describes the Law of Conservation of Linear Momentum?

• A. The momentum of the system increases with time.
• B. Momentum is always zero in an isolated system.
• C. The total momentum of the system remains constant.
• D. Momentum changes only during high-energy collisions.

Answer: (C)

Momentum is the product of mass and velocity, and it behaves nicely in a closed system: the total momentum stays the same. If no external forces act, the impulses on all parts of the system sum to zero, so the vector sum of all momenta remains constant before and after any interaction, including collisions. This is the essence of the law: the total momentum of the system remains constant.

The other statements don’t fit because momentum doesn’t have to grow over time, and an isolated system can have any nonzero total momentum depending on its initial state. Momentum can change if external forces act, not just during collisions, so it’s not limited to “high-energy” events.