Newton's law of acceleration applies to which condition of flight?

• A. Unbalanced
• B. Balanced
• C. Steady-state
• D. Static

Answer: (A)

Acceleration arises only when the forces acting on the aircraft are not in balance. Newton’s second law tells us that the net external force causes a change in velocity, scaled by the aircraft’s mass. In steady, level flight the lift equals weight and thrust equals drag, so the forces cancel and there’s no acceleration. But when you change thrust, or pitch to increase lift (or reduce drag), you create a net force that causes the aircraft to speed up, slow down, climb, or descend. So the law applies to conditions where the forces are unbalanced, which is when acceleration occurs.