For greatest safety in the event of a crash, a car is designed with a crushable front end and restraints to hold the riders to their seats.
While moving, the car and rider have momentum, defined as product of mass and velocity. A force is needed whenever a moving mass is to be stopped, or decelerated. In situations involving collision, it is more convenient to think of impulse: that which changes the momentum of an object. Impulse is defined as product of average force and elapsed time.
When a heavy car is stopped, an impulse acts on it to bring its momentum to zero value. The amount of impulse needed is exactly equal to the amount of momentum the car had originally; the impulse amount is independent of the manner in which the car is stopped. However, if the car is stopped suddenly in a collision (rather than gradually with the brakes), the elapsed time is small resulting in a large average force.
The crushable front end of the car serves to extend the time duration of the collision so as to lessen the force. But the rider has inertia and will keep moving once the car is stopped, until he/she strikes something that can deliver an impulse on him/her.
If a rider is not wearing a harness, he/she would likely be stopped suddenly by the steering wheel, dashboard and windshield. These are not crushable (the person more-likely is) so the impulse would be a combination of a short time and a very large force; injury or even death is the result.
The use of proper restraints, a harness and an airbag, would save the rider from serious injury.
Combined with the crushable front end, the harness delivers a force over time to stop the lower body. The upper body is stopped over time by the airbag; since less force is needed, injury is lessened.
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