By definition, Newton's First Law, also called the Law of inertia, indicates that "Every body perseveres in its state of rest or of uniform rectilinear motion unless it is forced to change its state by forces impressed on it."
This means that for a body to come out of its state of rest or of uniform rectilinear motion, it is necessary for a force to act on it.
So, in other words, all bodies are opposed to changing their state of rest or motion and this opposition is called inertia.
In this way, a body is in equilibrium when the resultant of the forces acting on it is zero. That is, it is not possible for a body to change its initial state (be it rest or motion) unless one or more forces intervene.
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