Airbags suppose to reduce injury?

[sp00ky]

I have a question about the airbags that goes into cars. 1. How are airbags suppose to reduce injury?? I know that it is something relating to Newton's second and third law, but I can't seem to find a way to explain it. Can anyone help me out?


Also, why would some people consider airbags dangerous?? Seems safer to me. :P

 

[nolachrymose]

You might want to read up on the conservation of momentum (which relates to the second and third laws quite easily).

 

[arildno]

The airbag has two important functions:
1) You're practically guaranteed that your head won't bang into the dashboard in a collision.
The collision between your head and airbag will last longer, but produce the same change in in your head's momentum; hence, the average force acted upon it will be lower. In addition, the contact area your head has with the airbag will, in general be larger than that if you smash your head into the dashboard; this will again lower the local presssure on your face (hopefully, to a value beneath that causing ruptures).
2. The relative motion betweeen your upper body and the rest is reduced, so that the possibility of spinal injuries is reduced.

From what I know, I think the main criticism against the airbag, is that you may suffer a backlash-effect; if you're unlucky, your head might be flung backwards, causing your neck to break.

 

[Alkatran]

The airbag has two important functions:
1) You're practically guaranteed that your head won't bang into the dashboard in a collision.
The collision between your head and airbag will last longer, but produce the same change in in your head's momentum; hence, the average force acted upon it will be lower. In addition, the contact area your head has with the airbag will, in general be larger than that if you smash your head into the dashboard; this will again lower the local presssure on your face (hopefully, to a value beneath that causing ruptures).
2. The relative motion betweeen your upper body and the rest is reduced, so that the possibility of spinal injuries is reduced.

From what I know, I think the main criticism against the airbag, is that you may suffer a backlash-effect; if you're unlucky, your head might be flung backwards, causing your neck to break.

Whenever I think about the dangers of an airbag, I think about a little old lady hunched over with her head an inch from the steering wheel, driving slowly...

 

[arildno]

I happen to think that that little old lady is more of a danger to herself by driving than any airbag ever will be..

 

[Sirus]

Spooky, what arildno said is correct. To elaborate on his first point: it's all about reducing the force acting on your head. We know $F_{net}=ma$. Therefore $F~\alpha~a$ when mass is constant, meaning that the greater the acceleration, the greater the force. Now we also know that
$$a=\frac{\Delta{v}}{\Delta{t}}$$
so
$$a~\alpha~\frac{1}{\Delta{t}}$$
meaning that if the change in velocity is constant (which it is if we take two identical collisions, one with and airbag, one without), the greater the time during which you are accelerated the smaller the magnitude of the acceleration. Now since we have already established that $F~\alpha~a$, we can conlude that
$$F~\alpha~\frac{1}{\Delta{t}}$$
Therefore, the greater the time during which you are accelerated (I guess decellerated in this case, although that is just negative acceleration), the smaller the force that acts on you. Now since you are accelerated over a greater period of time when an airbag inflates between your head and the steering wheel, we can conclude that a smaller force acts on you.

Some may be tempted to say "yes, but it acts on you for a longer time, so there is no difference", but keep in mind that it's the instantaneous force that we are concerned with. A small force will not hurt you even if if acts on you for a year, because it will never reach a high enough magnitude to do damage to your body.

A very interesting question.

Sirus

 

[russ_watters]

Something left out is the force is also spread out over a larger area - your entire head and torso hit the airbag.

The dangerous thing about airbags is the force of the deployment itself.

 

[arildno]

Something left out is the force is also spread out over a larger area - your entire head and torso hit the airbag.

The dangerous thing about airbags is the force of the deployment itself.

I did, as a matter of fact, mention that, but it was great that you emphasized that again:

It is the value of applied local PRESSURE which determines what types of injuries you're actually going to get.

 

[sp00ky]

You might want to read up on the conservation of momentum (which relates to the second and third laws quite easily).

Where do I find the thread of momentum?

 

[Sirus]

I think nolachrymose meant in a textbook or on the internet.

Russ_watters and arildno, I agree, the surface area is the other large factor influencing the level of injury, and surface area is maximized by an air bag.

 

[sp00ky]

i c i c... so in conclusion, would you consider airbags as a good and useful tool for cars or very dangerous?

 

[Sirus]

Airbags are most definitely good and useful tools: look at crash test results. Huge improvement. Airbags are an essential feature (no new car comes equiped without them), and the more the better.

 

[NoTime]

i c i c... so in conclusion, would you consider airbags as a good and useful tool for cars or very dangerous?

They are extremely dangerous if you are a small person.
This is why they prohibit children from the front seat.
It doesn't get any better if you are an old small person.

Airbags have been shown to save lives.
They have also been shown to kill people.

The newer systems, starting around 2000, are slightly better in this regard.

Statistics are great. Your personal experience may vary.

 

[Sirus]

You are correct in that the danger always exists, but probability wise (yes, this is based on stats) you are still less likely to be seriously injured in a vehicle equipped with airbags than in one without them. You are also correct in saying the newer systems are better at reducing risk for small people. Many new vehicles are now equipped with "smart" airbags that can either be set to various levels of deployment force or have mass sensors in the seat that adjust the force of deployment accordingly. This problem is virtually non-existant with side- and head-airbags.

 

[NoTime]

You are correct in that the danger always exists, but probability wise (yes, this is based on stats) you are still less likely to be seriously injured in a vehicle equipped with airbags than in one without them. You are also correct in saying the newer systems are better at reducing risk for small people. Many new vehicles are now equipped with "smart" airbags that can either be set to various levels of deployment force or have mass sensors in the seat that adjust the force of deployment accordingly. This problem is virtually non-existant with side- and head-airbags.

Are your conclusions based on any real world stats or just the test dummy stuff?

I know the "smart" airbags are being developed, had not heard that they were actually available.
Looking on the web I saw a projection for the 07 model year.
"Smart" as of the 04 model seems to mean they turn them off if you don't match the design (test dummy) spec.

 

[Sirus]

http://www.nhtsa.com/airbags/pages/FAQsAdvFrontABs.htm#1

I believe that confirms most of what we were saying. Yes, airbags can be dangerous, particularly to children, but the net probability of injury is still lower.