## How many g's was I subjected to ? I should not have survived.

I recently was in a head on collision. I am just a dumb fireman that needs an answer about my survival. I would like to know how many g's I was subjected to in my crash. They checked the event data recorder in my 2011 Ford Fusion and found that I was going 42 mph and in the same instance I went 4 mph backwards. The police said I should not have survived but here I am. I weight 175 lbs. The other car was about 600 pounds more than mine. I was belted in and the air bag did deploy. I am suffering a concussion and although I never lost consciousness I lost about 8 minutes of memory. I just would really appriciate an answer to help me cope. Thanks George
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Welcome to PF;
 I was going 42 mph and in the same instance I went 4 mph backwards
You cannot go backwards and forwards in the same instant... but you said "instance" so I don't know what you mean.

Do you mean the vehicle was travelling at 42mph and later went backwards as a result of the collision?

To know the acceleration the car underwent, we need to know the time-frame of the collision or have some idea about the distances involved... collisions generally last a lot longer than they appear to.

You should know that people regularly survive quite horrific crashes at much higher speeds than this. Modern vehicles are designed so that the energy of the collision is channeled around the cabin. As a result, it is common for rescue workers to see the damage to the car and marvel that the occupants survived.

Were you the driver?

You usually have about a yard between you and the wheel ... so, if you did not hit your head, it would have come to a stop in that distance from an initial 42mph. That would be about 20m/s over about 1m. Which average about 20gees ... so you probably either hit your head or blacked out[1] (or both) and woke up in time to get rescued. Certainly explains the concussion - you'll also have a bunch of aches and pains elsewhere.

Note: Short duration shocks of over 100g have been survivable (recorded in race-cars). A lot depends on the details of what happened ... you can get killed falling off a stool if you land funny.

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[1] maybe not - lateral (forwards/backwards) g-forces are much more survivable that vertical ones which cause blackouts due to pulling blood away from the brain.
http://en.wikipedia.org/wiki/G-force
http://en.wikipedia.org/wiki/Shock_(mechanics)
 Recognitions: Gold Member There is no easy way to figure the G's you experienced. Everything from the bumper of the car, to the frame, to the safety features such as airbags and seat belts are all designed to absorb energy and make the deceleration as long as possible in order to reduce the force applied to the human body. Even different parts of the body can decelerate at different rates. For example, your head isn't decelerated at the same rate as the rest of your body. It can be greater or less depending on if you have an airbag or not. As for crashes, my dad plowed a Semi-truck into the far side of creek, head on into the bank at around 60 mph or something. He walked away from the crash with some bruises and cracked kneecap. The first responding officer said he had never seen an accident that bad that someone walked away from it.

## How many g's was I subjected to ? I should not have survived.

Thanks for the reply. Yes, I was going forward and then 4 mph backwards. You are correct about the crush zone on the car. As you can see from the picture the passenger compartment stayed intact. The police only talked about my survival after a very long investigation. I guess they ran all the numbers,weights,marks in the road, etc. before they gave me that conclusion. I was the driver and I was alone in my vehicle. The other driver crossed the yellow solid line and we collided.I felt awful we both got hurt but there was nothing I could do. My head must have hit the airbag but I did have a cut on my forehead. I actually have gone to many car accidents for my job but I have never been on this side of the situation. Cars are quite remarkable compared to the old days. People were hurt much worse at lower speeds. I did have many aches and pains. I actually had bruises on my body from the seat belts within a very short time. Again thanks for the reply. 20 G's is alot of force.

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Welcome to PF!

 Quote by George M They checked the event data recorder in my 2011 Ford Fusion and found that I was going 42 mph and in the same instance I went 4 mph backwards.
Do you mean the next instance? What was the time interval between the recordings. This will give us the acceleration of the car*; yours would be somewhat less.

Glad you are [more or less] ok.

 *Er -- unless that is based on the rotation of the wheels only. Then it may not be accurate. I'm surprised the data recorder doesn't have an accelerometer.

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 Quote by George M The police only talked about my survival after a very long investigation.
Doesn't mean anything - the investigation was not focussed on the probability of survival. In NZ police always play up the change of dying because (a) they attend so many fatalities so have a skewed impression of crashes, and (b) to encourage safer driving in general.
 My head must have hit the airbag but I did have a cut on my forehead.
It is likely your head hit the airbag - the cut suggests you also hit something hard .. maybe the wheel, which implies the bag did not bring your head to rest, which reduces the longer-duration gee-force.
 I actually have gone to many car accidents for my job but I have never been on this side of the situation.
It's a revelation aye?
 I did have many aches and pains. I actually had bruises on my body from the seat belts within a very short time.
The bruises would have been deep too. You'll also have non-specific effects from your internal organs hitting the inside of your body.
 20 G's is alot of force.
But still inside the capabilities of the human body to survive.

BTW: it is unlikely that the cars hit dead-on ... so some of the energy of the collision would have gone into spinning around. I did not account for that, or the possibility that you stomped on the brakes before you hit so the speed right before the collision may have been smaller than 42mph.

With reference to the other replies - we really need the raw data from the recorder to do anything - I did the calculations in terms of your car. It is all very hand-wavey but you were not looking for rigor here. For instance: your head may have ended up stationary against the air-bag+wheel but the car may have still been moving at that time ... so the change in speed would have been less than 42mph. That 20gees is more of an upper bound.

The bottom line is that the crash, from the details available, is one we would expect to be survivable with injuries much like you describe. We are way off miracle territory here. Maybe the next ten guys who have that sort of crash will have worse injuries and a couple die ... that's probably the sort of thing the police mean.

I'm sure you have attended worse crashes where people have lived and crashes not so bad where people have died.

Scientists have to be hard-headed about these things - however small the chance of survival, given a lot of incidents, someone will survive some of them. The survivors will always wonder how it came to be that they lived and not the others. But somebody has to so ... why not you?

This sort of investigation can be useful in determining if there was something special about your crash ... which can help design better cars or roads or something. There may be nothing special either way ... in which case it was dumb luck and fecal matter materializes.

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 Quote by Simon Bridge You usually have about a yard between you and the wheel ... so, if you did not hit your head, it would have come to a stop in that distance from an initial 42mph. That would be about 20m/s over about 1m. Which average about 20gees
You need to look at initial velocity in center of mass frame, but in this case it works out to 44mph, which makes little difference. (Still just under 20m/s).

The travel distance is definitely higher, though. You get 1m from crumble zone alone. Another 0.5m or so thanks to the air bag. So we are looking at about 1.5m of travel, which drops it further to about 13G.

So it is certainly a survivable collision, so long as the belt and air bag function as they are supposed to. Evidently, they did.

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 Quote by Simon Bridge You usually have about a yard between you and the wheel ... so, if you did not hit your head, it would have come to a stop in that distance from an initial 42mph. That would be about 20m/s over about 1m. Which average about 20gees ... so you probably either hit your head or blacked out[1] (or both) and woke up in time to get rescued. Certainly explains the concussion - you'll also have a bunch of aches and pains elsewhere.
IIRC in the early days of seat belts and airbags, numbers in the range of 15 to 30g were used as a basis for inventing the first safety standards. Those numbers were for unrestrained movement of the occupants before the restraint systems started acting.

But estimating the actual accelerations in an unknown scenario is next to impossible. Crash tests are carefully controlled, and even then there can be a large scatter between the results of supposedly identical tests.

The object with the highest acceleration in your crash was almost certainly the airbag, not the car or yourself. Getting a face full of expanding airbag could easily give you concussion, but that's a lot better outcome than the alternatives!
 George, im glad your ok and I would like to say thank you. It's the everyday heros like you that dont get enough recognition for what you.guys do. Risking your own life to save others. Thank you.George.
 What happened to the other guy ?
 Thank You very much. We just love to help.
 She had more severe injuries than I. Broken bones and a bit more but I cant seem to find out all the details.

 Quote by CWatters What happened to the other guy ?
This was her car.
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 Thank you very much for the reply. I suspect she was drinking and that helps people quit a bit. They do not tense up and seems to be a huge fact in the accidents I have been to. One thing that I have seen happen is aortic tear from trauma . The aorta is damaged and starts to leak. You keep loosing blood until you go into shock. The patient starts off with normal blood pressure but just keeps decreasing until they have no more blood to pump. They die right in front of us. It is awful but does happen. I watched my b/p in the rescue and at the hospital and it was constant. They say if you are physically fit you have twice the chance of survival. Maybe this helped ? If I ever get the police report I will post all the specifics. I am very curious. I'm so happy I found this forum. You guys are great. Posting sure beats pushing up daisies.
 Recognitions: Homework Help 20+g was routine for Saturn V though the astronaughts blacked out. rocket sled tests show that 40g is survivable... I don't know the proportion that blacked out... but this is why I doubted the report of being conscious throughout. It is probable that some time was spent unconscious and the period is simply not remembered... I'd be surprised to learn that help arrived right away and he was lucid throughout. high g in short duration is more survivable than long duration and you stay conscious for higher g's laterally than vertically (you pass out because blood pressure changes in your brain in that case)... but you handle vertical compression better than lateral. Your analysis of the crash is similar to mine ... the vehicles did not hit head on though it is hard to see how much rotation there was (the long damage down the left-side of the red car could just be due to the crumpling - you've seen this before right?) My immediate impression was that the crash is not on the driver's side ... but then I remembered that this is the USA. Crashes from 60-70kmph, with a stationary object, in NZ, are routinely survivable. The stats take a bit of hunting down ... and you have to crunch the numbers yourself - see: http://www.transport.govt.nz/researc...tatistics.aspx ... but that is not the same as saying that you can go into a crash like that expecting to get away with such minor injuries. I used to live next to a really bad intersection - there would be a crash like this one every few months for about a year (lights were installed but people ran them - it took cameras to stop it) and I was one of the first at the scene each time. People generally survived - they didn't usually get up and walk away but they weren't usually all that messed up either. When I moved to Auckland, I lived down a length of motorway that was called "the meatgrinder" by locals ... I was first at the scene to six multi-car smashes in six months. The speed limit back then (80's) was 80kmph so people routinely did 100-120kmph in all weathers. People did survive them but not in good shape and the odds were clearly against it. There's a concrete wall separating traffic now. Anyway - the result was that I had a personal interest in motor vehicle crashes. It's been a while since I actually did anything though and I understand the state of the art has improved a great deal. We do have to be careful to distinguish between something being survivable and having a reasonable expectation of surviving.

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 Quote by Enthalpy In the favourable symmetrical head-on crash, 40km/s or 50km/s is considered a well-survivable speed. I attended once a test at 70km/s, and I didn't imagine someone would survive.
Some numbers.

11 km/s - Escape velocity of Earth.
30 km/s - Earth's orbital velocity.
42 km/s - Maximum velocity of solar asteroid passing Earth's orbit.
73 km/s - Maximum possible impact velocity of retrograde solar asteroid.

No, I don't imagine anyone surviving 70km/s car impact. I don't imagine anyone within a few hundred meters surviving an equivalent of an 0.8kT explosion that follows either.

Yeah, I'm sure it was a typo. But it's the consistency of that typo that made me chuckle a little.