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 .
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 crushable front end of the car serves to extend the time duration of the collision so as to lessen the force . 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.