How do seatbelts relate to the first law of motion

 in motion with the same speed and in the same direction unless acted upon by an unbalanced force

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. All objects resist changes in their state of movement. Without an unbalanced force, an object in motion will maintain its state of movement. This is often called the law of (slow or no movement/the force of something moving). Think about/believe for instance the crash of a car with a wall. Upon contact with the wall, an unbalanced force acts upon the car to suddenly slow down it to rest. Any passengers in the car will also be slowed down to rest if they are strapped to the car by seat belts. Being strapped tightly to the car, the passengers share the same state of movement as the car. As the car speeds up, the passengers speed up with it; as the car slows down, the passengers slow down with it; and as the car maintains a constant speed, the passengers maintain a constant speed also.