The law of inertia, also known as Newton's First Law of Motion, states that an object at rest stays at rest and an object in motion stays in motion at a constant velocity unless acted upon by a net force.

So, the answer to your question is: when the forces acting upon an object are balanced (or when there is no net force).

Here's why:

* Balanced forces: When forces on an object are balanced, they cancel each other out. This means there is no net force acting on the object.

* Unbalanced forces: When forces on an object are unbalanced, there is a net force acting on the object. This net force will cause the object to accelerate, meaning it will either start moving from rest or change its velocity if it was already in motion.

Let me know if you'd like more examples or explanations!