Use this as a quick reference for inertia, equilibrium, and directional independence.
🧭 Plot Summary
Newton's First Law is sometimes called the Law of Inertia, and that name gives away the whole idea: objects resist changes to their motion. An object at rest stays at rest. An object moving at constant velocity keeps moving at that same velocity. The only thing that can change either situation is a nonzero net external force. When all forces on a system perfectly cancel — ΣF = 0 — the system is in translational equilibrium and its velocity does not change. The key twist in this lesson is that equilibrium applies independently in each direction.
The critical nuance — directional independence
Forces can be perfectly balanced horizontally — constant vₓ.
While unbalanced vertically — vy changing. Both can be true at the same time.
What you'll do in this lesson
- State Newton's First Law and apply it to objects at rest and objects moving at constant velocity.
- Define inertia as a system's resistance to changes in velocity.
- Recognize translational equilibrium: ΣF = 0 in every direction.
- Apply the First Law independently to horizontal and vertical directions.
- Explain that velocity only changes in the direction of a nonzero net force.
- Identify an inertial reference frame as the perspective from which the First Law holds.
Why it matters
Newton's First Law sets the baseline every other dynamics problem is measured against. "Why is this object accelerating?" — because something changed that would otherwise be constant. "Why is this object in equilibrium?" — because ΣF = 0 in every direction. Understand the First Law deeply and the Second Law is just the quantitative version.
✅ Self-Check Before You Roll On
Check off each item as you get there. These aren't grades — they're your own signal.