What principle explains why a person keeps moving forward in a car that suddenly stops?

The principle that explains why a person keeps moving forward in a car that suddenly stops is inertia. Inertia is a property of matter that causes an object to resist changes in its state of motion. According to Newton's first law of motion, an object in motion will remain in motion at a constant velocity unless acted upon by a net external force.

When the car stops abruptly, the force that acted to stop the vehicle does not apply to the person inside it immediately. As a result, the person's body continues to move forward due to inertia, which is why they will lurch forward when the car halts.

Understanding inertia is crucial because it helps explain a range of everyday phenomena, such as why seatbelts are important for safety; they provide the necessary force to counteract the inertia of the person, helping to prevent injury during sudden stops. Other options, like friction, gravity, and momentum, relate to movement and forces but do not specifically capture this resisting motion scenario as effectively as inertia does.