Momentum and the rate of change of momentum principle is used in several real time applications. Airbag is one such application where the rate of change of momentum and subsequent reduction in impact force-principle is used for the safety of the passenger in automobile cars. Whenever the car is moving at high speeds there exists very high momentum associated with the car body. When a braking force is applied to stop the car, depending on the time of application of the brake force, the impact of the force will vary. Generally during the impacts and subsequent stoppages of the car, very high forces will act for a short duration of time and hence the impact force acting on the passengers will be very high. Inorder to extend the time of action of the forces air bags will be used. When an airbag hits the passengers in the body, the time duration of the force application will increase and limit the impact force to smaller quantities and hence the human body remains safe. Almost all the new generation cars are employing the air bags to provide safe impact forces to the passengers whenever the car comes in contact with the high magnitude impact forces.
Airbag usually consist of flexible fabric material made external surface and it will inflate in very little time when the automobile undergoes impact or any type of collision. This immediately provides cushioning effect to the occupants by preventing the impact and providing the protection to the bodies by preventing the direct impact with the steering wheel or a window of the car. Airbag with the combination of the seatbelts provide additional support to the passenger from getting impact on. Generally the compressed cylinders will be initiated in the case of impact and the air bags will get inflated when there is collision(Fukaya et al,2008).
Airbags when fist introduced few decades back, they are designed to protect the driver of the car but in the later times all the passengers of the car are designed to get protection from the impacts during the collision of the car.
The principle is readily understandable from the following equation:
I = F*t = m *(V-u)
I is the impulse, F is the impact force, t is the time of action of the force, m is the mass of the car and the occupants, V is the final velocity of the vehicle,(which will be zero) and u is the initial velocity of the car before the collision process.
Physics in air bag inflation:
Eventhough the exact timings of the airbag functioning varies from design to design, the exact values of the air bag inflation timings will be as follows, It usually takes about 20 milli-seconds for the crash sensors to sense the collision and to initiate the inflation process. Also the crash sensors do take around 25 milliseconds to decide the airbag inflation process. Also it will take about 20 milli-seconds for the person to land into. After 20 milli- seconds the person landed into the airbag the airbag starts deflation.
If T1 is the time of impact force transmission during a collision force and T2 is the time of impact force transmission with the air cushion. More and more the T2 than T1, lower and lower will be the impact force acting on the driver or the passenger.
Air bag is a passive working device, which does not require any positive action by the passenger during the collision of the automobile. However still they are considered as supplement device to the active safety devices like seat belts. Air bags are employed both for the body protection as well as for the knee protection of the passengers at present in several standard brands of the automobiles. The exact shape of the inflated airbag, the fabric material used for the airbag and the location of the inflation devices like cylinders do change from one model to another. However still the airbags are popular in almost all types of the cars and are in use throughout the globe. Now days using Airbags has become mandatory by the governmental legislations in several countries including united states of America. As high as 8 safety air bags simultaneously are also in use in several popular automobile brands now a days. Inflation of the airbags can be in as little time as one fifth of a second as well the deflation can also be fast through the holes made in the bag surface. One important requirement of the airbags is the lubrication during the collision process. In most of the cases, the lubrication employed during the inflation of the air bags is some sort of talcum powder, which ideally provides lubrication during the inflation process of the bag. Apart from the pressurized gas cylinders, other type of inflation initiators used with the airbags include controlled chemical reactions resulting in expansion of the airbags. Most commonly the combustion of the sodium azide, which releases nitrogen gas to inflate the airbag. Frontal collisions are the most common type of incidents in number of countries, the usage of the airbags in the frontal side found to avoid the fatalities however still there are few associated implications like facial and ocular incidents to the passengers in the car as reported in the accident incident investigations(Braver et al, 2004)(Buckley et al,1999).
The first and foremost requirement for the air-bags to operate effectively and to reduce the accident probability is to have proper crash sensors in the automobile. The time taken for the detection of the collision impact should be as low as possible. Since the collision can happen in any direction, the collision arrestors, air bags should also be mounted in all possible direction for complete protection (Bunker et al, 1997). Air bags mounted in frontal protection do arrest collision from the frontal impacts, collisions from the sides and from other curtain holders are other possible directions of mounting the airbags. Whatever may be the inflation mechanism used, the operation of the inflation should be very quick and the air bags should become operational very fast. The lubrication employed for the airbags should function fast to facilitate the free movement of the airbags. Generally air bags are employed as SRS (Supportive restraining systems), they function in combination with the seat belts. Hence the seat belts the active restraining system like seat belts should be functional (should be active) for the SRS bags to work on to protect the passengers (Molia et al, 1996).
However still there should be research and development required for customizing the airbags for the protection of the children and infants (Graham et al, 1998). However still in several practical observations of the collision and subsequent damage to the passengers, it is found that they are left with visible facial and ocular damages, they are usually caused by the high speed inflation of the airbags during the collision. Still further research and development is needed to reduce these inflation related injuries to the passengers during the collision process (Molia et al, 1996) (Giguere et al,1998)(Pearlman et al, 2001). Recent trends in the airbags as collision arrestors include the usage of the on/off switches which facilitates the user’s choice either to keep seat belts active or not, this is generally done keeping inview of the potential damage that the air bag can cause to the kids and infants during the collisions (Hayashi et al, 1995). In any ways correct mounting and the correct shaping and flexible operational features are the key requirements for the seat belt to successfully operate and protect the passenger from fatal injuries during the collision.
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