Which of Newton's laws explains why seatbelts are needed?

The need for seatbelts is best explained by the First Law of Motion, also known as Newton's First Law or the Law of Inertia. This law states that an object at rest will remain at rest, and an object in motion will continue in motion at a constant velocity unless acted upon by a net external force.

In the context of a car in motion, when the vehicle suddenly stops (due to braking or a collision), the passengers inside the car will continue to move forward at the same speed due to inertia. If seatbelts are not worn, the passengers may be propelled forward, potentially leading to serious injury or ejection from the vehicle. Seatbelts provide the necessary external force to secure the occupants and prevent them from continuing in motion when the car stops, effectively reducing the risk of injury during sudden stops or collisions.

While the Second Law deals with the relationship between force, mass, and acceleration, and the Third Law talks about action and reaction forces, they do not directly address the necessity of restraints for occupant safety in vehicles as clearly as the First Law of Motion does. The Law of Gravitation pertains to the attraction between masses and is unrelated to the dynamics of car safety.