In Newton's first law, what is meant by "net external force"?

In the context of Newton's first law, "net external force" refers to the total force acting on an object that can lead to a change in its state of motion. When we talk about the net external force, we are considering all the forces acting on the object from outside of it. If this net force is not zero, it can cause the object to accelerate, decelerate, or change direction, indicating that there is an effect on the object’s motion.

For example, if a person pushes a stationary book across a table, the push is an external force that affects the book's motion. If the push is strong enough to overcome any opposing forces (such as friction), the book will start moving. This illustrates how a net external force acts to change an object's state of motion, aligning with the principles described in Newton's first law, which states that an object at rest stays at rest, and an object in motion continues in motion at a constant velocity unless acted upon by a net external force.