Newton's First Law and Equilibrium

The law comes from inertia, the tendency of any object with mass to resist changes in its velocity. A hockey puck sliding on fresh ice keeps gliding because almost no friction acts on it horizontally, so there is nothing to slow it down. People expect a force to be needed to keep something moving because everyday experience is full of friction and air resistance, forces that constantly fight motion and make it look like pushing stops the instant you stop pushing. Take those resisting forces away and the object just keeps going, forever, at the same speed in the same direction.

The key idea is net force, the vector sum of every force acting on an object. Equilibrium means that sum is zero. That covers two different-looking situations: an object at rest, and an object moving at constant velocity in a straight line. Both count as equilibrium because in both cases velocity is not changing. A crate sliding at steady speed across a floor is just as much in equilibrium as a book sitting still on a table, even though one is moving and one is not.

This topic sets up everything else in the unit. Free-body diagrams are the tool for checking whether forces balance, and equilibrium problems are the simplest case before you add acceleration in the second law. Once net force stops being zero, the object accelerates, which is exactly where Newton's second law picks up.