Car crashes are one of the most common causes of
injury and death in the world. According to the World Health Organization,
about 1.35 million people die each year as a result of road traffic accidents.
To prevent or minimize the damage to the occupants of a car during a collision,
various safety features have been developed and implemented, such as seat
belts, crumple zones, and airbags.
In this article, we will focus on how airbags work and
how they reduce the impact force on a person during a car crash.
What is an airbag and how
does it work?
An airbag is a flexible fabric bag that inflates
rapidly when a car crashes, creating a cushion between the person and the hard
surfaces of the car, such as the steering wheel, dashboard, or windshield. The
airbag system consists of three main components: a sensor, an inflator, and the
bag itself.
The sensor is a device that detects when a car crash
occurs and sends an electrical signal to the inflator. The sensor can measure
various parameters, such as the acceleration, deceleration, or pressure of the
car, and trigger the airbag when a certain threshold is exceeded.
The inflator is a metal canister that contains a
chemical propellant that produces a large amount of gas when ignited by the
electrical signal from the sensor.
The gas flows into the bag through a nozzle and
inflates it in a fraction of a second. The chemical propellant used in most
airbags is sodium azide (NaN3), which decomposes into sodium metal (Na) and
nitrogen gas (N2) when heated. The reaction can be written as: 2NaN3(s) → 2Na(s)
+ 3N2(g)
The bag is made of nylon or polyester fabric that is
coated with silicone or urethane to make it airtight. The bag has small holes
or vents that allow some of the gas to escape as the person hits the bag,
reducing the pressure and preventing the bag from bursting2. The size and shape
of the bag vary depending on its location and function in the car. For example,
frontal airbags are usually larger and rounder than side airbags, which are
thinner and longer4.
How does an airbag reduce
the impact force?
To understand how an airbag reduces the impact force
on a person during a car crash, we need to review some basic concepts of
physics, such as momentum, impulse, and force.
Momentum is a measure of how much motion an object
has. It is calculated by multiplying the mass of the object by its velocity.
For example, if a person has a mass of 70 kg and is traveling at 20 m/s in a
car, their momentum is 70 kg × 20 m/s = 1400 kg·m/s. Momentum is conserved in
any isolated system, meaning that the total momentum before an event (such as a
collision) is equal to the total momentum after the event.
Impulse is a measure of how much momentum an object
gains or losses during an event. It is calculated by multiplying the average
force applied to the object by the time interval during which the force acts.
For example, if a person hits an airbag with an
average force of 5000 N for 0.1 s, their impulse is 5000 N × 0.1 s = 500 N·s.
Impulse is equal to the change in momentum of the object5.
Force is a measure of how much an object pushes or
pulls on another object. It is calculated by dividing the change in momentum of
an object by the time interval during which the momentum changes.
For example, if a person changes their momentum by 500
N·s in 0.1 s, their force is 500 N·s / 0.1 s = 5000 N. Force is proportional to
the acceleration of an object5.
Now let’s apply these concepts to a car crash
scenario. Suppose a person with a mass of 70 kg is traveling at 20 m/s in a car
that crashes into a wall and stops instantly. If there is no airbag, the person
will continue moving forward until they hit the wall or another hard surface in
the car. Assuming that their momentum changes from 1400 kg·m/s to zero in 0.01
s (a very short time), their impulse will be -1400 N·s (negative because their
momentum decreases) and their force will be -1400 N·s / 0.01 s = -140000 N
(negative because their direction changes). This means that they will
experience a very large and sudden force that can cause severe injuries or
death.
However, if there is an airbag, the person will hit
the bag instead of the wall or another hard surface. The airbag will inflate
and deflate quickly, creating a softer and longer contact with the person.
Assuming that their momentum changes from 1400 kg·m/s
to zero in 0.1 s (a longer time), their impulse will be the same as before
(-1400 N·s), but their force will be much smaller: -1400 N·s / 0.1 s = -14000
N. This means that they will experience a much smaller and gradual force that
can reduce the risk of injury or death.
As you can see, the area under the force-time curve
represents the impulse, which is the same in both cases. However, the peak force
is much higher when hitting a wall than when hitting an airbag. The airbag
reduces the peak force by increasing the time of contact, which decreases the
acceleration and deceleration of the person.
What are the benefits and
limitations of airbags?
Airbags have proven to be effective in reducing
fatalities and injuries in car crashes, especially when combined with seat
belts. According to the National Highway Traffic Safety Administration (NHTSA),
frontal airbags saved 50,457 lives from 1987 to 2017 in the United States6.
Side airbags also reduced the risk of death by 37% for drivers and 52% for
passengers in side-impact crashes7.
However, airbags are not perfect and have some
limitations and drawbacks. For example, airbags can cause injuries themselves,
such as burns, bruises, abrasions, or fractures, especially if the person is
too close to the bag or not wearing a seat belt8. Airbags can also malfunction
or deploy inappropriately, such as in minor collisions or when there is no
occupant in the seat. Airbags can also be expensive to replace after
deployment.
Therefore, it is important to follow some safety tips
when using airbags, such as:
·
Always
wear a seat belt and adjust it properly.
·
Sit at
least 10 inches away from the steering wheel or dashboard.
·
Do not
place any objects or children on your lap or near the airbag.
·
Do not
install any aftermarket devices or covers on the airbag.
·
Check
the airbag warning light and have it serviced if it stays on or flashes.
·
Follow
the manufacturer’s instructions and recommendations for your car and airbag
system.
Conclusion
Airbags are one of the most important safety features
in modern cars. They work by inflating rapidly when a car crashes, creating a
cushion between the person and the hard surfaces of the car.
They reduce the impact force on a person by increasing
the time of contact, which decreases the acceleration and deceleration of the
person.
They have saved thousands of lives and prevented many
injuries in car crashes, but they also have some limitations and risks.
Therefore, it is essential to use them correctly and responsibly, along with
seat belts and other safety measures.