If no net force acts on it, an object at rest remains at rest and an object in motion remains in motion with a constant velocity. Newton's First Law is also called the Law of Inertia. Inertia is the ability of an object to resist changes in its state of motion. The more mass an object has, the more inertia it has and the harder it is to change its motion. An object with a lot of inertia takes a lot of force to start or stop; an object with a small inertia requires a small amount of force to start or stop.