If the external net force on an object is zero, the object will remain at rest or continue to move at a constant velocity (uniform motion) A net force is not required for an object to maintain a constant velocity. A net force is required to change the velocity of an object in magnitude, direction, or both. External forces are required to change the motion of an object. Internal forces have no effect on an object"s motion. Inertia: a measure of an object"s resistance to change in velocity. An object at rest will stay at rest unless a net force acts on it. If an object is in motion, it will maintain a constant velocity unless a net force acts on it. The degree to which an object resists a change in motion depends on the magnitude of the object"s mass objects that have more mass have more inertia. Mass: a measure of the amount of matter in an object.