State Newton's first law, identify equilibrium situations, and explain motion using the concept of inertia.
For over a thousand years, most thinkers assumed that objects naturally slow down and stop unless something keeps pushing them -- it certainly looks that way with a rolling ball on a rough floor. Newton's first law corrects this: an object at rest stays at rest, and an object in motion stays in motion at constant velocity (same speed, same direction), UNLESS acted on by a net (unbalanced) outside force. The ball actually slows down because of friction acting on it, an outside force -- not because motion "runs out" on its own. This resistance to a change in motion is called inertia, and it depends only on an object's mass: more mass means more inertia, meaning more resistance to being sped up, slowed down, or redirected.
A net force of zero does not mean NO forces are acting -- it means all the forces acting on an object cancel out, called equilibrium. A book resting on a table has two forces on it (gravity pulling down, the table pushing up), and because the book is not accelerating, those two forces must be equal in size and opposite in direction. Equilibrium is one of the most useful ideas in all of engineering: bridges, buildings, and hanging signs are all designed so that every force balances to zero.
Mastery looks like: They identify the forces in a simple equilibrium situation, show that they must balance to zero, and explain everyday motion using the idea of inertia.
Common stumbles: Believing a moving object needs a continuous force just to keep moving at constant velocity; assuming equilibrium means no forces are present rather than balanced forces.