Newton's First Law: an object at rest stays at rest, and an object in motion stays in motion at constant velocity in a straight line, unless acted on by a net external force.
Inertia is an object's tendency to resist any change in its state of motion — whether that means starting to move, stopping, or changing direction. Newton's First Law is sometimes called the Law of Inertia for exactly this reason.
Inertia depends on mass: the more mass an object has, the more it resists a change in motion. A loaded shopping cart takes more effort to get rolling — and more effort to stop — than an empty one, purely because it has more mass and therefore more inertia.
Everyday experience seems to teach that moving objects always slow down and stop unless you keep pushing them. That intuition is understandable — and it's exactly backwards.
Give an object a one-time push (setting its initial velocity), then release it — no further applied force, ever. Toggle friction on and off and watch what velocity actually does.
Without friction, the line stays perfectly flat forever — no force is needed to sustain constant velocity. With friction, an actual backward force is doing the work of slowing the object down. Nothing "runs out."
It's worth separating two words people often use interchangeably. Mass is the amount of matter in an object and the source of its inertia — it doesn't change no matter where the object is. Weight is the force of gravity pulling on that mass (F_g = mg), and it changes depending on the strength of gravity wherever the object happens to be.
Amount of matter. Source of inertia. The same on the Moon as on Earth.
Force due to gravity. Smaller on the Moon, where gravity is weaker, even though mass hasn't changed at all.
Equilibrium is easy to spot when something is obviously sitting still. It's much easier to miss when something is clearly moving.
A car cruises down a flat highway at a perfectly steady 30 m/s. Is it in equilibrium? What does its free-body diagram look like?
Newton's First Law tells you what happens when forces balance: nothing changes. The natural next question is what happens when they don't — when the net force isn't zero. That relationship, connecting force, mass, and acceleration directly, is exactly where Unit 2 goes next.