Key Concepts: Newton’s First Law

Newton's First Law (Law of Inertia)

An object at rest stays at rest, and an object in motion stays in motion with the same speed and direction, unless acted upon by a net external force.

• Inertia: The resistance of an object to changes in its state of motion. More mass = more inertia.
• If the net force on an object is zero, its velocity does not change (it either stays still or moves at constant velocity).

Equilibrium

• Static equilibrium: Object is at rest and ΣF = 0.
• Dynamic equilibrium: Object moves at constant velocity and ΣF = 0.

Everyday Examples

• A passenger lurches forward when a bus brakes suddenly — their body's inertia keeps them moving forward.
• A hockey puck slides on ice (very low friction) and barely slows down.
• Objects in deep space, far from gravitational influences, travel in straight lines at constant speeds indefinitely.