Final answer:
The primary reason safety belts are important is to stop passengers' forward motion by spreading the force over a stronger part of the body over a longer time, which reduces force through the physics concept of impulse.
Step-by-step explanation:
The primary reason that safety belts are important to a driver in a crash is because a body in motion tends to stay in motion according to Newton's first law of motion. In the event of a crash, safety belts work to stop the forward movement of the driver and passengers by spreading the stopping force across a stronger part of the body over a longer period of time. This mechanism is tightly related to the concept of impulse, which is the product of the net force applied to an object and the time over which the force is applied. The longer time to slow down reduces the force experienced by the occupants, contributing to less severe injuries.
Cars that crumple or collapse upon impact are designed to increase the time over which a collision occurs, effectively reducing injury to the passengers by increasing the time of impact and subsequently decreasing the net force. The crumple zones in a vehicle absorb some of the impact's energy, so that it's not directly transferred to the passengers.
Both the crumple zones of a car and the use of safety belts demonstrate how engineering, informed by the laws of physics, is crucial in improving road safety and minimizing injuries. By extending the time over which a collision occurs, whether through the structural design of the car or the restraint system, the impulse is spread out, and the force exerted on passengers is significantly lowered.
For example, during a collision if a driver is restrained by a seatbelt, the force exerted on them will cause their velocity to decrease to zero over a longer period of time compared to if they were unrestrained and hit a rigid surface directly. This use of physics knowledge in safety feature design is why modern vehicles are much safer compared to older models.