Newton's First Law: The Law of Inertia
* The natural state of motion of a force-free system is a state of uniform motion in a straight line.

* An object moving at a constant speed needs no net external force to keep it moving; its inertia keeps it moving at a constant speed in a straight line.

* If the sum of all the forces acting on a system is zero, meaning that the magnitudes of the forces cancel each other out, then the net force on the system is zero. The system will then move at a constant velocity in a straight line if it is moving, or remain at rest if it is at rest.

* If either the magnitude or direction of an object's velocity changes, then there must be some force impressed upon the object. This is actually the definition of when there is a non-zero force acting on an object.

* For a system to be in a state of mechanical equilibrium, the vector sum of all the forces acting upon the system must add up to zero.