The "Perfect" Crash
Obviously, the ideal crash would be no crash at all. But, let's assume you are going to crash, and that you want the best possible chances of survival. How can all of the safety systems come together to give you the smoothest crash possible?
Surviving a crash is all about kinetic energy. When your body is moving at 35 mph (56 kph), it has a certain amount of kinetic energy. After the crash, when you come to a complete stop, you will have zero kinetic energy. To minimize risk of injury, you would like to remove the kinetic energy as slowly and evenly as possible. Some of the safety systems in your car help do this.
Ideally, your car has seatbelt pretensioners and force limiters; they both tighten up the seatbelts very soon after your car hits the barrier, but before the airbag deploys. The seatbelt can then absorb some of your energy as you move forward towards the airbag. Milliseconds later, the force in the seatbelt holding you back would start to hurt you, so the force limiters kick in now, making sure the force in the seatbelts doesn't get too high.
Next, the airbag deploys and absorbs some more of your forward motion while protecting you from hitting anything hard.
In this hypothetical crash, the safety systems in the car all worked together to slow you down. If you didn't wear your seatbelt then the first stage of your protection is lost and it is going to hurt a lot more when you slam into the airbag. Many cars have seatbelt pretensioners and force limiters, but there are some even more exciting safety improvements coming.