Air Bags are Life Savers


Changing an object's momentum requires a force acting over a period of time.

If momentum changes in a very small time period, such as in a car crash, then the force is very great. If the momentum can be changed over a longer period of time, even a fraction of a second more, much less force needs to be applied and this results in less damage to the car or injury to the occupants.

Air bags are made from a strong coated fabric. They are stored in a module mounted on the steering wheel and dashboard and side panels of the car. The inflation of them is initiated by crash sensors  that activate upon impact at speeds of more than 10-15 miles per hour. They are mounted in several locations on the car body. In a crash the sensor sends an electrical signal to the air bag which then causes the air bag to deploy. It ignites a chemical propellant which produces nitrogen gas, this then inflates the bag itself.

As the driver or passenger' head and chest is thrown into the bag, it slows him/her down more gradually than would be the case with the bag not there. When the air bag is set off and is out to it’s full size, there are vents at the rear which allow air to slowly be removed from it. This is what cushions the head when an air bag works. As the head strikes the bag, it forces air out the vents at the back which allows for the head to sink into the pillow of air and increases the time over which the change in momentum of the passenger occurs. The change in momentum is determined by the speed of travel. It cannot be changed, but the force at which the person comes to a stop decreases as the time factor increases. Even though this entire process happens in only 1/25th of a second, the added time is enough to prevent serious injury.

Air bags do not just reduce the impact force by elongating the time factor, they also spread the impact over a larger contact area. By doing this, the force is not all concentrated in one small area of your body and the pressure on your body is reduced. This in turn will cause the seriousness your injuries to be reduced.

They sure go 'bang' when they are set off - it might make you think you have been shot!

Also see seat belts and crumple zones they work on the same principle.

Ft = impulse

Ft = Δp = mΔv

Always make it clear to the examiner that you understand that the safety feature does not 'reduce momentum'. That depends on the speed of the vehicle!

Explain that it causes the time factor in the impulse to increase and that makes the force factor decrease.