The net force on a system is simply the vector sum of every individual force exerted on it — exactly what your free-body diagram from Lesson 2.2 already shows you, added together.
When that vector sum comes out to exactly zero, the system is said to be in translational equilibrium:
Try building your own force system below — three adjustable forces, shown both from a free-body-style common origin and as a tip-to-tail vector chain. When the chain closes back on itself, the system is in equilibrium.
Adjust three forces acting on a system. The left diagram shows them from a common origin (like a free-body diagram); the right shows the same vectors added tip-to-tail. If the chain closes back to its starting point, the system is in equilibrium.
Newton's first law states that if the net force exerted on a system is zero, the system's velocity will remain constant.
Equilibrium is checked axis by axis, not just as a single overall yes-or-no. Forces can be perfectly balanced along one direction while completely unbalanced along a perpendicular direction — and a system's velocity will change only along the unbalanced direction, staying constant along the balanced one.
Try it directly below, using the same block-on-track scenario from Lesson 2.2's free-body diagram tool.
The block-on-track scenario from Lesson 2.2. Adjust the four forces — watch how the two axes are judged completely independently.
Try setting the sliders so one axis is balanced and the other isn't — this is exactly the situation described in 2.4.A.4.
A block slides down a rough incline at a constant velocity. What does this tell you about the net force on the block, both along the incline and perpendicular to it?
An inertial reference frame is one from which an observer would actually verify Newton's first law — a frame where objects with zero net force genuinely appear to move at constant velocity, rather than mysteriously accelerating for no apparent reason.
Unless a problem explicitly states otherwise, you may assume every reference frame in this course is inertial — the same boundary assumption introduced back in Lesson 1.4.