Seat Belt and Air Bag

Seat belt: According to Newton's First Law, there will be inertia on the driver when the car is moving. If the car is brought to complete stop in a very short instance during a car crash, the driver would tend to "fly off" in the direction of the original displacement, due to inertia. The seat belt's function is to "lock" the driver (overcome intertia) so that he won't fly off during the crash.

Air Bag: The air bag can make the time of impact longer so that the damage to the driver would be reduced. (Impact = F / time)
F is calculated using Newton's 2nd law (F=ma)
Nothing about action and reaction.

At the moment of crash in a traffic accident, the driver and the passengers in a vehicle tend to move forward due to inertia. Seat belt is used to keep the driver and the passengers in their seat during the collision.

However, the driver may also hit the steering wheel during the collision even when he wears the seat belt. Therefore, airbag was invented and is installed inside the steering wheel of a car so that it will inflate in a fraction of second at the moment of crash. Thus, the upper body of the driver will hit the airbag during crash instead of the hard steering wheel so that the degree of injury of the driver will be greatly reduced. In luxury cars, side airbags are also installed in the A and B columns of the cars to minimize the injuries of the driver and passengers resulted from the side impact in traffic accident.

By F=(v-u)/t
v and u are constant
and t 越長 F就會越細
So the force act on your body 會細左，reduce 傷害