Definition
The law stating that a body at rest tends to remain at rest, and a body in motion tends to remain in motion in a straight line at a constant speed, unless acted upon by an outside force. Often called the Law of Inertia.
Plain English
Things don't change how they're moving on their own. Something at rest stays at rest, and something already moving keeps going the same way at the same speed, until a force pushes or pulls it differently.
Context Anchor
Seen in aerodynamics discussions about turns, where the airplane must be forced away from straight-line motion to follow a curved path.
Derivation
Named for Sir Isaac Newton, the English scientist who published these laws in 1687. 'Inertia' comes from the Latin iners, meaning 'idle' or 'sluggish' -- capturing the idea that matter resists changes to its motion.
Why Pilots Care
An aircraft will continue straight until the pilot applies control inputs and engine power to change its path or speed.
Analogy
A rolling suitcase keeps going straight unless you pull the handle sideways. The sideways pull is what changes its path.
Grounding Statement
An airplane does not curve through the sky just because the pilot wants it to; a sideways force must continuously pull it through the turn.
Intuition Check
Do not read this as "moving objects keep moving forever no matter what." The law means motion stays the same only when no outside force changes it.
Example Sentence 1
Newton's First Law of Motion explains why an aircraft in cruise will continue straight and level until a control input or outside force changes its path.
Example Sentence 2
Newton's First Law explains why an aircraft in flight tends to maintain its current velocity until external forces act on it.