How can an airbag soften a stop that happens in an instant?

When a vehicle stops suddenly in a crash, a passenger’s body tends to keep moving. The passenger must also slow down. An airbag helps manage that change in motion by providing a surface that gives way while spreading contact over part of the body.

The useful idea is stopping time and distance. For the same change in momentum, increasing the stopping time reduces the average force required. A hard surface can bring a moving body part to rest over a very short distance. A suitably positioned airbag can deform as the body presses into it, allowing a more gradual stop. Real forces vary throughout the crash, so the average is not the peak force.

The bag cannot wait for a person to hit it before starting to inflate. Sensors and a control unit evaluate a crash and, when deployment conditions are met, trigger an inflator. Gas quickly fills the folded bag. The exact decision depends on the vehicle, crash direction and severity, and the restraint system’s design; a pothole is not treated as a qualifying crash.

As the occupant loads a conventional vented frontal airbag, gas escapes through designed openings. The bag can give way rather than acting like a rigid inflated wall or returning all the motion like a spring. Its fabric, gas flow and placement help shape the stopping process. Different airbag types use different designs.

An airbag is part of a restraint system, not a substitute for a seat belt. The belt helps position and slow the occupant while the bag supports particular regions in the crash it was designed for. Rapid inflation itself can injure someone too close to the module. The remarkable engineering is coordinating a controlled, brief interaction among the car, belt, bag and moving body.