Seatbelts apply Newton's first law, also known as the law of inertia, in a practical and life-saving manner. Newton's first law states that an object at rest will stay at rest, and an object in motion will continue moving at a constant velocity in a straight line unless acted upon by an external force.
When a vehicle suddenly decelerates or comes to an abrupt stop, the passengers inside it tend to continue moving forward due to their inertia. In the absence of any external force, such as a seatbelt, their bodies would keep moving forward even if the vehicle has come to a halt.
Seatbelts function by providing the necessary external force to counteract the forward momentum of the passengers. When a seatbelt is properly worn, it applies a restraining force to the person's body. As a result, it prevents the passenger from continuing to move forward at the same velocity as the vehicle.
According to Newton's first law, the passenger's body naturally resists any change in its state of motion. Therefore, the seatbelt effectively overcomes this resistance by exerting a force in the opposite direction of the passenger's motion, ultimately bringing the person to a stop along with the vehicle.
By applying Newton's first law, seatbelts help protect occupants during sudden decelerations or collisions by ensuring they remain inside the vehicle and reducing the risk of injury.