Newton's first law of motion states that An object at rest stays at rest and an object in motion stays in motion with the same speed and in the same direction unless acted upon by an unbalanced force. Objects tend to keep on doing what they're doing. In fact, it is the natural tendency of objects to resist changes in their state of motion. This tendency to resist changes in their state of motion is described as inertia. Inertia: the resistance an object has to a change in its state of motion.