Which law is demonstrated when a rocket expels gas backward and moves forward?

• A. First Law of Motion
• B. Second Law of Motion
• C. Conservation of Energy
• D. Third Law of Motion

Answer: (D)

Newton's third law of motion is illustrated: for every action there is an equal and opposite reaction. When the rocket expels gas backward, the gas is pushed in the backward direction, and the gas pushes the rocket forward with a force of the same magnitude in the opposite direction. This forward push, called thrust, drives the rocket ahead. If external forces are negligible, the system’s total momentum stays the same, so the forward momentum of the rocket increases as the backward momentum of the exhaust increases. Inertia or the idea that force equals mass times acceleration doesn’t by itself explain why the thrust occurs, and conservation of energy describes energy changes rather than the direction of motion; the propulsion arises specifically from the action-reaction pair between the rocket and its expelled gas.