What principle does the law of inertia demonstrate in aviation physics?

The law of inertia, formulated by Sir Isaac Newton, states that an object in motion tends to stay in motion, and an object at rest tends to remain at rest unless acted upon by an external force. In aviation physics, this principle highlights the necessity of external forces, such as thrust, drag, lift, and weight, to change the state of motion of an aircraft.

For example, when an aircraft is flying straight and level, it will continue to do so due to its inertia. If the pilot wants to climb, descend, or turn, they must apply an external force through the use of the aircraft's controls (such as adjusting the throttle or ailerons) to overcome inertia and change the aircraft's motion. This illustrates the fundamental role of external forces in modifying the velocity or trajectory of an airplane, reflecting the core concept behind the law of inertia.