🧭 Plot Summary
In 2-1-1, you learned that when every force on an object balances out — equilibrium — the net force is zero. Today that idea becomes Newton's First Law: an object at rest stays at rest, and an object moving at constant velocity keeps moving at that same velocity, unless a net force acts on it. Both are the same statement. Rest is just the constant-velocity case where the velocity happens to be zero.
This law also defines inertia — an object's built-in resistance to having its motion changed. The more mass something has, the more inertia it has, and the harder it is to start moving, stop moving, or change direction.
What you will do in this lesson
- Formalize the equilibrium idea from 2-1-1 into Newton's First Law of Motion.
- Define inertia and connect it directly to mass.
- Take down the oldest misconception in physics: that motion needs a continuous force to keep going.
- Learn to spot equilibrium in situations that don't look like 'nothing is happening.'
- Preview the difference between mass and weight, ahead of a full treatment later.
Why it matters
Almost every dynamics problem you'll ever solve starts by deciding whether an object is in equilibrium or not. Get comfortable recognizing equilibrium today, and every remaining lesson in Unit 2 gets easier.
✅ Self-Check Before You Roll On
Check off each item as you get there. These are not grades — they are your own signal.