Holding the mass of the car and the total change in velocity constant (as when braking you will always be breaking from a set speed), the impulse for the car will be constant as well, because I=mΔv
When a car breaks suddenly, as the non-seat-belt-wearing passengers have a tendency to maintain their path (inertia), they are flung forward until they collide with the front; at this point, a force acts on them to slow them down along with the car
Crumple Zones
When 2 cars collide, the crumple zone contracts (absorbing the impact force) and thus a retarding force is applied on the car through a larger amount of time.
Thus, as impulse is constant; as (t) increases, the retarding force applied to the car decreases, thus the damage done to humans inside the car is decreased
Airbags
When a passenger experiences a car crash; the air bag increases the amount of time it takes for the passenger’s head to experience the retarding force.
Thus for a constant impulse, the time over which the force is experienced increases, the force applied decreases
Seatbelts
All objects have inertia; a tendency to maintain their state of motion
When a collision occurs our body will also maintain their state of motion and continue to travel forwards if no unbalanced force is applied to oppose this motion
A seat belt is used to apply an unbalanced force in the opposite direction of the motion, causing our bodies to decelerate rapidly; preventing us from crashing out the wind screen