Newton’s First Law

Statement

A body remains at rest or in uniform motion in a straight line unless acted upon by a net external force.

Mathematical Form

$$ \sum \vec{F} = 0 \quad \Rightarrow \quad \vec{v} = \text{constant} $$

Interpretation

• If no net force acts, velocity does not change
• Rest is a special case of uniform motion

Inertia

The tendency of a body to resist changes in its state of motion is called inertia.

• Greater mass $\rightarrow$ greater inertia