Which of Newton's laws explains why a passenger lurches forward when a car stops suddenly?

Newton's first law of motion explains why a passenger lurches forward when a car stops suddenly. The law, also called the law of inertia, says that an object remains at rest or continues moving in a straight line at constant speed unless acted on by a net external force.

When the car is moving, the passenger and vehicle travel forward together. If the driver brakes abruptly, the car slows because braking forces act on the vehicle and its tyres. The passenger's body tends to keep moving forward at its original speed. The seat belt then supplies the force that slows the passenger and keeps them with the car.

This is not an example of a mysterious forward force pushing the passenger. It is inertia: resistance to a change in motion. In everyday language, people may say the passenger is “thrown forward,” but the important physics is that the car stops more quickly than the unrestrained body does.

Galileo developed the underlying idea of inertia before Newton formalized it in the laws published in the 1687 Principia. Seat belts apply the needed force over a controlled distance and help prevent serious injury.