How do airbags reduce injury using momentum principles?

Airbags reduce injury by increasing the time over which the momentum of the passenger changes, reducing the force.

When a car crashes, the passengers inside are still moving at the car's original speed. According to the principle of momentum, an object's momentum is the product of its mass and velocity. To stop the passengers, their momentum must be brought to zero. This change in momentum is what can cause injury if it happens too quickly.

Airbags work by inflating rapidly during a collision, providing a cushion that slows down the passenger more gradually. This increases the time over which the passenger's momentum changes. According to Newton's second law, force is equal to the change in momentum divided by the time over which the change occurs (F = Δp/Δt). By increasing the time (Δt), the force (F) experienced by the passenger is reduced.

For example, if a passenger's momentum changes from a high value to zero in a very short time, the force would be very large, potentially causing serious injury. However, if the same change in momentum occurs over a longer period, the force is much smaller, reducing the risk of injury.

Additionally, airbags spread the force over a larger area of the passenger's body compared to, say, a seatbelt alone. This further reduces the pressure on any single part of the body, minimising the risk of injury to specific areas like the chest or head.

In summary, by increasing the time over which the momentum change occurs and spreading the force over a larger area, airbags significantly reduce the forces experienced by passengers, thereby reducing the likelihood and severity of injuries in a crash.