Question

why should seat belts be slightly extensible

Answer 1

When a car is moving with a high velocity, its momentum is also high and the people in the vehicle are also in the same momentum. ... So when a car meets with an accident, the person falling front with the presence of stretchable seat belts as these elastic seat belts reduce the momentum in more time to reduce the force.

Answer 2

Answer:

When a car is moving with a high velocity, its momentum is also high and the people in the vehicle are also in the same momentum.  So when a car meets with an accident, the person falling front with the presence of stretchable seat belts as these elastic seat belts reduce the momentum in more time to reduce the force and it does not cause an injury to the person.