Which statement about momentum is true?

• A. Momentum is not influenced by external forces.
• B. Momentum is conserved in a closed (isolated) system.
• C. Momentum always increases in a collision.
• D. Momentum is measured in joules.

Answer: (B)

Momentum is mass times velocity, with units of kg·m/s. In a closed or isolated system where no external forces act, the total momentum remains constant because the forces during interactions are internal and cancel out in total. During a collision, the objects exchange momentum, but the sum of their momenta before equals the sum after. That’s why the statement about momentum being conserved in a closed (isolated) system is the true one. External forces can change momentum through impulse, so momentum isn’t always unchanged in every situation. Also, momentum is not measured in joules (that is energy, not momentum). For a helpful picture, imagine two carts on a frictionless track pushing off each other: they may speed up or slow down individually, but the combined momentum stays the same.