FAA Flight Instructor Airplane (FIA)Aerodynamics and Principles of FlightEasy
An airplane flying straight and level at a constant altitude and airspeed demonstrates Newton's first law of motion because:
- AThe four forces are perfectly balanced, so the airplane continues in uniform motion without acceleration
- BThe airplane's mass changes continuously as fuel burns, altering its equilibrium
- CThrust exceeds drag slightly, causing a constant, gentle acceleration
- DGravity is momentarily suspended due to the wing's lift production
Show answer & explanationAnswer & explanation
Correct answer: A. The four forces are perfectly balanced, so the airplane continues in uniform motion without acceleration
Newton's first law (the law of inertia) states that an object in motion remains in uniform motion unless acted upon by an unbalanced force. In steady, unaccelerated flight, lift equals weight and thrust equals drag, so the net force is zero and the airplane continues at constant velocity, illustrating this law directly.
Why the other options are wrong
- B. Fuel burn changes mass gradually but does not describe Newton's first law directly.
- C. If thrust exceeded drag, the airplane would accelerate, violating the 'constant airspeed' condition.
- D. Gravity is never suspended; it is balanced by lift, not eliminated.
Newton's First Law in Flight
An airplane in steady, unaccelerated flight has balanced forces (lift=weight, thrust=drag), so it continues at constant velocity per Newton's first law of inertia.
- Newton's first law: object at constant velocity unless acted on by net force
- Balanced forces = zero net force = no acceleration
- Any imbalance (e.g., thrust>drag) causes acceleration
Memory trick: Balanced forces = boring, steady flight (inertia at work)