Which statement represents the Law of Counterforce?

• A. No external force is needed
• B. Energy cannot be created or destroyed
• C. Momentum is conserved in all cases
• D. For every action there is an equal and opposite reaction

Answer: (D)

Newton's third law says forces come in action–reaction pairs: when one object pushes on a second, the second pushes back with the same magnitude in the opposite direction. The statement that for every action there is an equal and opposite reaction captures this idea precisely. This is why you feel a pushback when you push against a wall, or why a rocket moves forward as it expels gas backward. On a larger scale, walking or jumping relies on pushing against the ground so the ground can push you forward. The other ideas mix up different concepts: you do not need an external force to have a counterforce, but the interaction involves two objects; energy conservation is about energy, not the pairing of forces; and momentum conservation isn’t guaranteed in all situations since external forces can change the total momentum of a system.