If one person were to let go, however, the force on the rope would become unbalanced. This unbalanced force would cause the rope to move. An object at rest will remain at rest unless it is acted on by a net, external force. This statement is half of Newton's First Law, or the Law of Inertia. They have been verified by several centuries of testing and observations.
Force is a vector.
This means that a force entails both a value and a direction. To determine the total net, external force acting on an object, it is necessary to determine all forces acting on the object, and then perform vector addition. For instance, a person sitting on a chair is experiencing a downward force due to gravity. He is also experiencing, however, an upward force due to the chair. These two forces, when added using vector addition, cancel each other out, giving no net, external force.