Definition
The principle, often called the law of inertia, stating that a body at rest remains at rest, and a body in motion continues in motion in a straight line at constant speed, unless acted upon by an outside force.
Plain English
Things don't start moving, stop moving, or change direction on their own. Something has to push or pull them to make that happen.
Context Anchor
Seen in aerodynamics and aircraft performance discussions when explaining how forces change an airplane's speed, direction, or attitude.
Derivation
Named after Sir Isaac Newton, the English physicist who published these laws of motion in 1687. 'Inertia' comes from the Latin iners, meaning 'inactive' or 'sluggish' -- capturing the idea that objects resist changes to their current state of motion.
Why Pilots Care
Pilots must anticipate that an airplane will continue its current motion until control inputs or external forces such as drag, lift changes, or wind alter it.
Analogy
A cart on a flat floor will sit still until someone pushes it. Once it is rolling, it tends to keep rolling until friction, a brake, or another push changes what it is doing.
Grounding Statement
Picture an airplane rolling straight ahead on a smooth runway: it will not speed up, slow down, or turn unless some outside force changes its motion.
Intuition Check
Do not think of this law as saying that motion always needs a continuous push. It says motion changes only when an outside force changes it; in real airplanes, air resistance is one of the forces that must be dealt with.
Example Sentence 1
By Newton's First Law, the aircraft sitting on the ramp will not move until the thrust from the engine overcomes the friction holding it in place.
Example Sentence 2
When the pilot advances the throttle, the added thrust becomes the external force that increases the airplane's forward speed.