According to Newton's first law, an object in motion continues in motion with the same speed and in the same direction unless acted upon by an unbalanced force. This is often called the law of inertia. Inertia is the tendency of a body at rest to remain at rest or of a body in straight line motion to stay in motion unless acted upon by an external force.
If the car were to abruptly be brought to a stop by a collision and seat belts were not being worn, then the passenger in motion would continue in motion and most likely be propelled from the car due to the inertia. Once they leave the car, the passenger becomes a projectile and continues in projectile-like motion.
