Physics and Car safety

The Use of the Airbag

The purpose of an airbag is to help the passenger in the car reduce their speed in  collision without getting injured.
Objects in a car have mass, speed and direction. If the object, such as a person, is not secured in the car they will continue moving in the same direction (forward) with the same speed (the speed the car was going) when the car abrubtly stops until a force acts on them.
Every object has momentum. Momentum is the product of a passenger's mass and velocity ( speed with a direction). In order to stop the passenger's momentum they have to be acted on by a force. In some situations the passenger hits into the dashboard or windshield which acts as a force stopping them but injuring them at the same time.
An airbag provides a force over time. This is known as impulse. The more time the force has to act on the passenger to slow them down, the less damage caused to the passenger.


There is a restricted amount of time that the airbag has to act between when the car hits the other object and the passenger hits the steering wheel.
  • About 15 to 20 milliseconds after the collision occurs the crash sensors decide whether or not the collison is serious enough to inflate the airbag (usually 6 - 10 km/h).
  • If the crash sensors decide to inflate the airbag it will be deflated at about 25 milliseconds after the crash.
  • It takes about 20 milliseconds to inflate the airbag for the person to land into.
  • Around 60 milliseconds the person has made contact with the airbag and the airbag now starts to deflate.
  • The passenger continues to be acted on by the airbag as it is in the deflation process which takes about 35 to 40 milliseconds.


It is still necessary to wear a seatbelt although all automoblies must be equiped with an airbag because of a few reasons:

1) The crash sensors do not signal for the airbag to inflate unless the vechile is moving atleast 6 km/h. Damage can still occur to the passenger if the collision is of a slower speed. This is were the seatbelt plays an important role.

2) The airbag located in the steering wheel does not help the passenger in a collision where another car hits them side on.

3) When the car the passenger is in is backing up and collides the its rear end with another object the airbag does not help.

With today's technology other airbags are being introduced  in addition to the steering wheel airbag and the passenger airbag. Such airbags that are being introduced are side airbags and head airbags.


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