Grade 11 · Physics
Inertia & the First Law
Why it matters
Balanced forces and equilibrium.
Core20m readingPrerequisite: None
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.
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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.