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The Physics of Airbags

~Gabby Gazall

Introducing Airbags
At the beginning invention stages of airbags, there were two main obstacles to overcome to increase safety protection: first, the need to establish a restraint system, and secondly, to create a 40 millisecond or less inflation. Since then, airbags have incorporated seatbelts to act as an additional counteracting force that is paired with the function of airbags. Over the years, researchers have worked diligently to manipulate the positioning of airbags for the most effective outcome of protection and they have discovered that using seatbelts serve as a critical function of drivers’ protection when used in combination with airbags.external image airbagexplose.jpg
The Physics of Airbags Introduction
Airbags themselves are quite simple and triggered to inflate through a chemical reaction when in contact with enough impact to register a response. They are designed to open up at over 200 mph—faster than a car can crash—to counteract before the time the driver absorbs the full contact of the crash. However, it is this very process of how airbags prevent the impact of the crash and its entirety, which is strategically and impeccably accurate and directly related to physics. The function of airbags and their reduction of injury are formed through the series of laws of motion, force, and impulse, which plays a vital role in the amount of protection that the driver will receive.
Law of Motion
A fundamental law of physics involves the concepts of mass and kinetic energy (energy in motion). As a general rule, the heavier an object, or the more mass an object has, and the greater the speed, the more kinetic energy it will have. In addition, the concept of velocity is speed, but specifically in reference to a certain direction. Thus, when all the energy in car.jpgmotion is suddenly stopped, the energy must be transferred to somewhere else. The drivers also have mass and velocity, which is why they continue to move even after the car has stopped. According to Newton’s first law of motion, an object in motion must stay in motion unless acted upon by a force of some kind. In the same way that a car is stopped when it comes in contact with another car, the law of motion applies to airbags, which provided a barrier to stop the driver. In both cases the car in motion was halted by an outside force as well as the driver who was stopped by the airbag in combination of with the seatbelt. The concept of an object that stays in motion and is only combated with an outside force is a crucial component to understand how airbags prevent injury.
Force
The seatbelt is indeed considered a means of force and inadvertently increases the driver’s safety. This is because it positions the driver, restrained to a certain point, to ensure that the airbag will come in direct contact with a specific area—the head. Although the car and the driver carry the same velocity when moving, the car bears much more mass than the driver and therefore has a greater force. In this way, when a car crashes, the force that impacts the automobile will also impact the driver. The seatbelt is a force that counteracts that impact and the airbags will additionally provide as much of a balanced force between the crash and the driver. Although the crash itself will have more force because the car has more mass, the airbag and the seatbelt act as a combined force to diminish the impact. A simplified drawing of the force vectors that are acting on the driver at the time of a crash is drawn below:
forceful.jpg

Impulse
Once the car stops, the energy is transferred from the car to the driver and the driver’s energy is transferred forward in the direction in which the car used to be moving. This energy transfer is momentum (mass times velocity) and is proportional to impulse (force times time). When time is decreased, the force is increased and vice versa. In the case of a car crash, the more time it takes for a car to come in contact with a barrier, the less forceful the blow. In the same way, airbags are triggered to immediately inflate, but the time it takes to inflate completely is reduced to prevent a greater impulse on the driver.carr.jpg
Rebounding
This involves the change in direction of a moving object as a result of a collision. Essentially there is a crash within a crash during an automobile accident—the car hits a barrier and the driver hits the airbag. In both instances, the car will “bounce” back and the driver will also rebound as an effect of the impact. This is important to increasing the amount of safety because without it, the impact of the crash would simply carry the driver through the windshield, but with less force. Changing the direction of motion proves to be the factor that saves the driver from continuing forward.
Putting It All Together
Air bags accomplish this idea of reducing injury by extending the time required to stop the momentum of the driver. When encountering a car collision, the driver will continue to move according to Newton's first law. The driver’s motion carries them towards a windshield during which the airbag inflates faster than the collision of the car and leaving room to absorb the force of the driver. Instead of a large force exerted over a short time, the airbag provides a 'cushion' in order to stop the momentum of the dirver. As soon as the driver hits an air bag, and the time duration of the impact is increased. When hitting an object with fluctuation’ like the air bag, the time duration might be increased. Increasing the time will result in a decrease in force, thus limiting the full impact of the crash on the driver. The driver’s body is absorbed in some regards in the airbag as it inflates, in addition to the seatbelt, which as more as a restrainer so the airbag can absorb as much mass and force in a given area. In this way, the driver’s head will not immediately rebound, as the energy and momentum is absorbed in the airbag and the seatbelt, rather it is absorbed then rebounds, thus shielding the driver from the full force of the crash. Now that's physics in action.

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Sources
http://www.physicsclassroom.com/class/momentum/u4l1c.cfm
http://www.bradylawgroup.com/practice-areas-defective-airbags-physics-function.php
http://www.nhtsa.gov/people/injury/airbags/airbags03/page3.html
http://auto.howstuffworks.com/car-driving-safety/safety-regulatory-devices/airbag.htm
http://wiki.answers.com/Q/Physics_behind_how_an_airbag_reduces_injury
http://www.caranddriver.com/features/the-physics-of-airbags-feature
http://www.ehow.com/about_5393997_history-airbags.html