An airbag expands in front of a driver's head in an accident.If there was no airbag, the windshield would have stopped the motion of his head in a shorter time.Compared to the windshield, the airbag exerts a smaller,larger or same force?
The equation force = mass X acceleration comes into play here.
Given that the driver moving into the windshield moves that greater distance in less time than the air bag moving into the driver travels the shorter distance, it's safe to say that when the driver is traveling at a much greater acceleration than if he/she were to hit into the airbag. Not only that, but the driver has much much more mass traveling into the windshield than the air bag has coming out at him.
Both of these factors make for WAY more force when the driver hits into the windshield than the other way.