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Why airbags work
Beginner
Longer stopping time reduces the average force for the same impulse.
An airbag lengthens the stopping time, so the same momentum change needs a smaller average force and so less injury.
Stopping 70 kg from 15 m/s in 0.05 s needs 21 kN; in 0.2 s only 5 kN.
Impulse stays the same; the force changes.
In more detail
An airbag increases the time over which a passenger's momentum changes to zero. The same change in momentum with a longer time produces a smaller average force.
Real-world uses
- Car makers design crumple zones and airbags to extend the stopping time.
- Helmets and cushioned mats reduce force by lengthening the time of impact.
- Airbags and crumple zones extend the stopping time of a crash, which reduces the average force on passengers.
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Frequently asked questions
Why do airbags reduce injury?
They spread the change in momentum over a longer time, so the peak force is lower.
Does an airbag change the momentum change?
No. The change in momentum is the same, but the time over which it happens is longer.
Why does catching a ball with soft hands hurt less?
Your hands move back as you catch, lengthening the time to stop the ball, so the average force is lower.
Related topics in Momentum & Collisions
Momentum is a vector · Impulse · Impulse–momentum theorem · Conservation of momentum · Isolated system · Recoil of a gun