In U.S., since the year 1999, car makers have been required to install both driver and front-passenger airbags. Airbags help prevent head, chest and other severe injuries during a car collision. The bag is so designed that it can inflate in less than a second after a car collides; thus, it serves as a cushion and stops the head and chest from hitting the hard surfaces in the car.
In a moving car, the passenger possesses a momentum which is the product of the person's mass and the velocity of the car. When a collision occurs, the passenger may hit the dashboard or the windshield, which acts as a force against it and can seriously injure the person. An airbag adds a cushioning force, known as "impulse," preventing the passenger from hitting the dashboard. According to Washington University, "the airbag must begin to deflate (i.e., decrease its internal pressure) by the time the body hits it," otherwise the pressurized air would create a hard surface.
The crash sensors of airbags take about 15 to 20 milliseconds to determine whether or not to activate the airbag. If sensors activate the airbag, it takes only about 20 milliseconds to inflate the airbag so the passenger can safely land into it.
How Airbags Work
An airbag system consists of a sensor that can recognize head-on collisions and immediately activate the inflating airbags. The sensor consists of a sliding steel ball. The ball is held by a permanent magnet which restricts the ball's motion when the car drives over uneven roads. But when a car collides, the ball moves very fast and turns on an electrical circuit. The electrical circuit ignites a pellet of sodium azide. This generates nitrogen gas which fills a nylon bag.
The airbag system is not fool-proof. The crash sensors signal the airbag to inflate only if the vehicle is moving at least 6 kilometers per hour. If a car is being hit side on, the front airbags do not help.
Role of Seat Belts
The time taken for an airbag to inflate depends on whether the seat belt is in use or not. Because too many people don't wear seat belts, airbags are designed to infate with explosive force, according to the Dr. Spock website. The force of inflation can cause serious injury to a person who is seated very near to the airbag. Due to such a risk, you should never install infant seats in the front seat because it would put the infant too close to the airbag.