Newton's First Law of Motion: The Law of Inertia

Newton's first law of motion states that an object at rest stays at rest, and an object in motion continues in uniform motion (constant velocity in a straight line) unless acted upon by an external net force. This principle is known as the law of inertia.

Key Concepts

• Object at Rest: A book on a table remains stationary until a force (e.g., pushing) moves it.
• Object in Motion: A rolling ball continues rolling until friction, a wall, or another force stops or redirects it.

Inertia

Inertia is the tendency of an object to resist changes in its state of motion. Mass quantifies inertia: more massive objects have greater inertia.

Real-World Context

In reality, forces like friction, gravity, and air resistance always act, causing eventual changes in motion. The law assumes an idealized frictionless environment. It explains phenomena like passengers lurching forward when a car brakes suddenly (their bodies tend to continue moving) and is fundamental to space travel where external forces are minimal.