The Naked Scientists

The Naked Scientists Forum

Author Topic: Why do seat belts reduce the rate of deceleration during a car accident?  (Read 11042 times)

Offline turnipsock

  • Hero Member
  • *****
  • Posts: 586
  • Beekeeper to the unsuspecting
    • View Profile
There is an ad on the telly telling us to wear a seat belt. It shows a man not wearing one and having a head on crash. They say the cause of death was his internal organs being thrust forward and hitting his ribs. Well, wouldn't his rate of deceleration be just the same if he was wearing seatbelt?

If you think about it, you would prably be better without the seat belt and then let the air bag decelerate you a little slower.
« Last Edit: 27/11/2008 07:58:18 by chris »


 

Offline DoctorBeaver

  • Naked Science Forum GOD!
  • *******
  • Posts: 12656
  • Thanked: 3 times
  • A stitch in time would have confused Einstein.
    • View Profile
I thought exactly the same when I saw it. What they are saying is that it is the sudden stopping of forward movement that kills as your internal organs keep moving - inertia. I don't see how wearing a seat belt would stop that. In fact, as you said, the seat belt would make the effect worse.
 

Offline RD

  • Neilep Level Member
  • ******
  • Posts: 8125
  • Thanked: 53 times
    • View Profile
The deceleration will be less with the seatbelt than hitting the steering wheel because the belt will stretch a little.
The seatbelt also spreads the force over a larger area than a steering wheel, so will do less damage than a steering wheel.
 
« Last Edit: 27/11/2008 05:05:44 by RD »
 

Offline DoctorBeaver

  • Naked Science Forum GOD!
  • *******
  • Posts: 12656
  • Thanked: 3 times
  • A stitch in time would have confused Einstein.
    • View Profile
The deceleration will be less with the seatbelt than hitting the steering wheel because the belt will stretch a little.
The seatbelt also spreads the force over a larger area than a steering wheel, so will do less damage than a steering wheel.
 

RD - that's not how it was shown in the ad. The driver didn't hit the wheel because the airbag deployed.
 

Offline RD

  • Neilep Level Member
  • ******
  • Posts: 8125
  • Thanked: 53 times
    • View Profile
Whether or not an airbag deployed, the force of the deceleration will be less with a beat belt than without,
 because of the stretching of the belt, (analogous to bungee jumping).
« Last Edit: 27/11/2008 06:37:36 by RD »
 

Offline dentstudent

  • Neilep Level Member
  • ******
  • Posts: 3146
  • FOGger to the unsuspecting
    • View Profile
As previously stated, it is the webbing in the seat belt that stretches and reduces the rate of accelaration on the wearer. It is also very important to: check the belts when you're buying a second hand car and; NEVER buy a second hand childs car seat - it may have been in an accident and the belts will already be stretched, therefore losing much of the safety aspects.
 

Offline DoctorBeaver

  • Naked Science Forum GOD!
  • *******
  • Posts: 12656
  • Thanked: 3 times
  • A stitch in time would have confused Einstein.
    • View Profile
So are you saying there is more give in a seat belt than in an airbag?
 

Offline dentstudent

  • Neilep Level Member
  • ******
  • Posts: 3146
  • FOGger to the unsuspecting
    • View Profile
Who, me?
 

Offline DoctorBeaver

  • Naked Science Forum GOD!
  • *******
  • Posts: 12656
  • Thanked: 3 times
  • A stitch in time would have confused Einstein.
    • View Profile
 

Offline dentstudent

  • Neilep Level Member
  • ******
  • Posts: 3146
  • FOGger to the unsuspecting
    • View Profile
Who, me?

Yes - and RD.

DB, I haven't mentioned airbags at all. All I said was how seat belts work.

However, there is a great deal of merit to wearing a seat belt. If it's working properly and locks as it should, then your own body has had a great deal of acceleration removed by being propoerly restrained. If the belt is loose, then in a crash, your body has time to gain inertia before the belt stops you which potentially causes a greater level of damage. I'm not saying that there won't be any damage if the belt is tight, but that there is likely to be a reduction. I'm sure that someone can show this with a simple bit of physics.

BTW, I haven't seen the ad, so I can only comment from my own experiences.
 

Offline Madidus_Scientia

  • Neilep Level Member
  • ******
  • Posts: 1451
    • View Profile
It may actually increase the rate of deceleration in the effort of stopping you from flying through the windscreen. And the seatbelt probably has more give than the truck or wall you would hit otherwise.
« Last Edit: 28/11/2008 08:49:45 by Madidus_Scientia »
 

Offline DoctorBeaver

  • Naked Science Forum GOD!
  • *******
  • Posts: 12656
  • Thanked: 3 times
  • A stitch in time would have confused Einstein.
    • View Profile
I think people are getting the wrong idea about this thread. Neither Turnipsock nor I is of the opinion that we shouldn't wear seatbelts.

The point being made is that there is an advert which runs along the lines of:-

John didn't want to die. He couldn't help it.
*man has car crash, no seatbelt being worn, airbag deploys*
It wasn't hitting the *whatever-it-was* that killed him. It wasn't *something else* that killed him.
Because he wasn't wearing a seatbelt his internasl organs continued moving forwards, breaking his ribs. His lungs were punctured and the main artery from his heart was torn open. That's what killed him.


The question Turnipsock is asking and is also puzzling me (and this is not the only forum I've seen this advert queried on; I've seen at least half a dozen) is whether your internal organs would continue to move forward due to inertia even if you were wearing a seatbelt. The ad shows the driver's chest hitting the airbag. WOuld there be less give in the airbag than in a seatbelt?

Here's a reply from 1 of the other forums:-

Quote
I am glad someone else has spotted this mistake - and I do say mistake.

The deceleration of the body in an accident is determined by the distance over which the body's speed reduces to zero. A shorter distance the greater the force and more likely the internal injury depicted in the advert. Yes, seat belts save lives, but not for the reason stated; they stop people being thrown out of a car or hitting a solid lump of metal or the windscreen. There is no way you can stop the sort of injury depicted by a seatbelt - in fact it will may it worse by reducing the distance in which the body comes to a stop and increasing the force on it.

I'm not saying that it's wrong to stress the importance of wearing seatbelts, just that the science behind this particular ad it seems questionasble.



 

Offline DoctorBeaver

  • Naked Science Forum GOD!
  • *******
  • Posts: 12656
  • Thanked: 3 times
  • A stitch in time would have confused Einstein.
    • View Profile
Stuart - I know you didn't mention airbags. You said about the way a seatbelt stretches a bit so I questioned whether you were inferring that stretching would be greater than the give in an airbag.
 

Offline dentstudent

  • Neilep Level Member
  • ******
  • Posts: 3146
  • FOGger to the unsuspecting
    • View Profile
Eth - no, there's no way that I would infer that the give in a belt is more than in a bag.

Perhaps the "marketing" behind this ad is to make people aware that just because you have an airbag, it may not necessarily save your life, and that it does not negate the use of seat-belts.

I would say that because the body has gained inertia due to not wearing a belt, then there is a greater proportional difference between the skeleton etc stopping and the internal organs, therefore potentially increasing internal injury. If the belt was being worn, there would still be a difference, but perhaps the amount would be reduced. The airbag would certainly provide a longer slowing period than the belt, and therefore a better disipation of the forces, but i would suggest that the start point from which this momentum has to be abated is considerably higher than it is for the seat belt wearer.

I also stress that this is purely a heuristic thought process and not based on evidence at hand.
 

lyner

  • Guest
It's all a matter of IMPULSE. - That's the change of momentum you need in order to stop.
Impulse is Force time Time.
Take twice as long and the force needed is halved. etc etc

I think the advert is a bit of poetic licence but hitting your head on a windscreen will bring you to a halt in a fraction of the time that a seat belt will take - hence a lot less force needed.

dentstudent
Quote
I would say that because the body has gained inertia due to not wearing a belt,
gained? Not really - he has not lost any momentum until hitting the bag.

You could say that the seatbelt will start the impulse and the bag will keep it going - more time - less force. How far does a seatbelt let you move in a head on impact? Does it let your chest get as far as the bag?

The advert could well just be bullshit, of course.
 

Offline DoctorBeaver

  • Naked Science Forum GOD!
  • *******
  • Posts: 12656
  • Thanked: 3 times
  • A stitch in time would have confused Einstein.
    • View Profile
I fully appreciate that hitting a steering wheel with one's chest, or a head hitting the windscreen, would still result in very serious injuries, but I still don't see how a seatbelt could stop the inertia of internal organs. I think the ad is wrong to say that wearing a seatbelt would have prevented the injuries portrayed. I've been in a low-speed head-on crash, wearing a seatbelt, and my innards got one hell of a jolt.

 

lyner

  • Guest
Like I said - seat belt plus bag will increase the time involved, which will reduce the force for the same impulse.
By 'inertia', I take it you mean 'momentum'. 'Inertia' is not a word which is defined particularly well - it can mean mass or massXspeed or massXvelocity, depending on who you are.
Or the ad could be plain wrong; I suspect that it may be.
Of course, a belt is much more use in keeping you in place in anything but a frontal impact so it is worth while encouraging its use.
 

Offline Bored chemist

  • Neilep Level Member
  • ******
  • Posts: 8655
  • Thanked: 42 times
    • View Profile
I may be missing something here but, if you stay in the car then, on average, your rate of deceleration over the course of the crash is the same. You and the car come to a stop at much the same time. If you stopped first the car would run into you, if the car stopped first you would run into it and mpove it slightly forward. You and the car must come to a halt at the same time.
The seatbelt's job must be to even out that deceleration (reducing the peak) and to spread the force over a reasonably large area (which is why it's a seatbelt, not a seat rope). If it also stops you getting thrown from that car that will help too.
 

lyner

  • Guest
BC
The front of the car may stop 'dead' (in a very few ms) but the rest of it will take longer (200ms, perhaps) as the front crumples. Because of the belt / bag, you will take even longer to come to a halt. So you do not stop 'at the same time' as the car'.
Same impulse needed to bring you to a halt but a much smaller force needs to act on you if you can extend your 'stopping time'..

A wide belt will involve a smaller pressure, agreed, and reduces the damage to any one particular rib,  for instance, but the acceleration (negative) of your body would be similar. The effect on your internal organs wouldn't be very different unless the one rib were to be pushing onto your liver, perhaps.
I am increasingly of the opinion that the advert could be flawed. It wouldn't be the first one. But, if it scares people into wearing seatbelts and driving carefully, then good luck to it.
 

The Naked Scientists Forum


 

SMF 2.0.10 | SMF © 2015, Simple Machines
SMFAds for Free Forums
 
Login
Login with username, password and session length