Newton's first law
High School
Definition
The principle that an object stays at rest or moves at a constant velocity unless acted on by an unbalanced (net) force. Also called the law of inertia, it explains why objects resist changes to their state of motion.
Worked examples
A hockey puck sliding on ice keeps moving in a straight line at constant speed until friction or a stick stops it.
No net force means no change in motion—the puck's inertia keeps it going.
A book resting on a table stays at rest because the upward normal force balances its weight, making the net force zero.
Balanced forces mean zero net force, so the object remains at rest.
Common mistakes
- An object needs a force to keep moving at constant velocity. → An object moves at constant velocity with zero net force. Newton's first law says motion continues without force—only acceleration needs force.
- Inertia is a force that keeps objects moving. → Inertia is the property of matter that resists changes in motion, not a force. Inertia describes resistance to change; forces cause the change itself.
- If an object is moving, there must be a net force acting on it. → Constant velocity means net force is zero; only acceleration requires net force. Force changes motion (accelerates); it doesn't sustain constant motion.
Where you'll use it next
Newton's first law is the foundation for analyzing motion with Newton's second and third laws, solving force and acceleration problems, understanding momentum, and studying orbital mechanics and reference frames in physics.
Found in 1 StudyPug lesson
Master Newton's First Law of Motion: The Principle of Inertia
11th Grade11thAS-Level Maths
Dive into the fundamental concept of inertia and its role in physics. Understand how objects behave at rest and in motion, and apply this knowledge to real-world scenarios.
See also
Reviewed by Pat Cheng, M.Ed. — StudyPug Curriculum Lead · Last updated June 6, 2026