State Newton's first law of motion (inertia) and use it to explain everyday demonstrations.
Newton's first law says an object at rest stays at rest, and an object in motion stays in motion at the same speed and direction, unless an unbalanced force acts on it. This resistance to a change in motion is called inertia -- more massive objects have more of it.
Inertia explains a lot of everyday surprises. When a car stops suddenly, your body keeps moving forward, not because something pushes you, but because your body's inertia keeps it doing what it was already doing until the seatbelt applies an unbalanced force to stop it too.
Mastery looks like: They state Newton's first law accurately and use it to explain a demonstration, naming what unbalanced force (if any) changed the motion.
Common stumbles: Calling inertia a force that pushes things; forgetting inertia keeps an object doing whatever it was already doing, including staying still.