When a force acts on an object that is moving, or able to move, its momentum changes.
Force = Change in momentum / time taken for the change
For a particular change in momentum, the longer the time taken for the change, then smaller the force.
In a collision, the momentum of an object often becomes zero during the impact - object comes to rest. If the impact time is short, the forces on the object are large. As the impact time increases, the forces become less.
Crumple zones in cars are designed to fold in a collision. This increases the impact time and so reduces the force on the car and the people in it.
Air bags work in a similar way. The driver's head changes momentum slowly when it hits an airbag. So the force on the head is less than if it changes momentum quickly by hitting the steering wheel.
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