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How does inertia affect the motion of passengers when a car stops suddenly?

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Title page of Isaac Newton's Philosophiae Naturalis Principia Mathematica, 1687, featuring large Latin text and publication details in a bordered layout. — Newton’s laws of motion The title page of Isaac Newton's *Philosophiae Naturalis Principia Mathematica* (1687; *Mathematical Principles of Natural Philosophy*), the work in which the physicist introduced his three laws of motion.

When a car stops suddenly, the motion of its passengers is governed by the law of inertia, which is the first of Newton’s laws of motion.

Inertia is the inherent property of a body that causes it to resist any change in its state of motion. Because the passengers are moving at the same velocity as the vehicle, their bodies have a natural tendency to continue moving forward in a straight line even after the car’s brakes are applied.123

If a passenger is not restrained, this inertia causes them to continue moving at the vehicle's previous speed, potentially leading to a collision with the interior of the car or ejection from the vehicle. To mitigate this, vehicular safety devices such as seat belts and air bags are used to provide the external force necessary to decelerate the passenger along with the car, thereby preventing injury.45