Physics 11 · Physics · Lesson 4.1
Inertia & the First Law
- Level
- Grade 11
- Reading time
- 12 min reading
- Review
- Last reviewed 2026-09-03
- Standard
- Accuracy standard
- Errata
Abstract
Balanced forces and equilibrium.
What is Newton’s first law of motion?
Short answer
Newton’s first law, the law of inertia, states that an object at rest stays at rest and an object in motion stays in motion at constant velocity unless acted on by a net external force. Inertia is the tendency of matter to resist changes in its motion, and it increases with mass. The law defines what a force does: change an object’s state of motion.
Concept preview
What you will be able to do
- 01State Newton's first law and define inertia
- 02Relate mass to inertia
- 03Identify balanced forces and equilibrium
Here's Newton's first law in plain English: things keep doing what they're doing. An object at rest stays at rest, and an object moving stays moving at constant velocity — same speed, same direction — unless a net (unbalanced) force shoves it out of that state. This 'leave me alone' stubbornness has a name: inertia. A hockey puck on perfectly frictionless ice would glide forever; on real ice it only slows down because friction is a sneaky unbalanced force.
School Plan
Inertia & the First Law
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Related topics
Frequently asked questions
- What is inertia?
- The tendency of an object to resist changes in its motion; the greater its mass, the greater its inertia.
- Does the first law mean no forces act on a moving object?
- It means no net force acts; balanced forces can still be present while the object moves at constant velocity.