View Full Version : A question for boffins.
A friend was involved in a car accident recently, she was the passenger in the front seat. The car was moving at approx. 25mph when it ran into the back of a stationary car, the result was the rear seat passenger, who weighed about 20 stones and wasn't wearing a seatbelt, was thrown forward into my friends seat. My friend suffered concussion from being propelled into windscreen, but thankfully is okay, as is the rear seat passenger.
The question is, what would the rear seat passenger's equivalent weight go up to due to G-force involved?
A friend was involved in a car accident recently, she was the passenger in the front seat. The car was moving at approx. 25mph when it ran into the back of a stationary car, the result was the rear seat passenger, who weighed about 20 stones and wasn't wearing a seatbelt, was thrown forward into my friends seat. My friend suffered concussion from being propelled into windscreen, but thankfully is okay, as is the rear seat passenger.
The question is, what would the rear seat passenger's equivalent weight go up to due to G-force involved?
Am i right in saying that the front passenger would be dead and possibly so would the rear?
No Vistravi, both are still walking about healthily.
The g-force is appox 100 times less the mass's deceleration on contact with the seat ,tranferable g would be 84 x 20 stone approx (not knowing the make ,model and impact site ...may change these figures )
A 1 month old Citreon C5 estate, I don't know the other car involved but it was a square on hit.
I thought this was going to be a joke until i read the last line.
I think I heard or read somewhere that the rear passenger would hit you with a force equivilent to 7 tons. Or I may have made it up.
I think I heard or read somewhere that the rear passenger would hit you with a force equivilent to 7 tons. Or I may have made it up.
A better question is what was the driver doing to crash square into the back of a parked car at 25MPH?
annemarie482
30-Mar-11, 15:47
A better question is what was the driver doing to crash square into the back of a parked car at 25MPH?
exactly my thoughts ducati!!
George Brims
30-Mar-11, 23:37
A better question is what was the driver doing to crash square into the back of a parked car at 25MPH?
The word was stationary, not parked. Someone who comes to a stop in front of you is stationary. Hitting them is still not smart. Sadly, speaking from experience, I can tell you it's also expensive and embarrassing.
The answer to the original question is that I can't tell you because I don't know how short a distance it took for the fat * in the back to go from 25 mph to zero. I can tell you if your friend had suffered injuries as a result of being hit by said FB, she would have been justified in suing.
*As a boffin I just conducted a serious experiment. Can you say "fat " on the org without getting in trouble? Results to follow...
Update: OK the result is the probably offending word gets removed. The word I combined with "fat" describes a Mike Myers character.
A better question is what was the driver doing to crash square into the back of a parked car at 25MPH?
Both cars had overtaken a bus which had stopped at a bus stop, the car in front had signalled to overtake the bus and left his indicator on because he intended to turn right about 20 yds further on, the driver of the rear car assumed the lead car was going straight on and had omitted to cancel his indicator; meanwhile, the driver of the car behind was distracted by his front seat passenger having difficulty fastening her seatbelt and doesn't see the lead car stop to allow oncoming traffic to clear, in that instant the collision happened.
The correct thing to have done was to make sure all passengers have their seatbelts on before driving off, then the accident wouldn't have happened.
No Vistravi, both are still walking about healthily.
Ah i think i thought you were meaning if the car was going faster and the same thing happened.
My partner has a friend who's car was mangled after an accident and he walked away without a scratch. Lady luck on his side that day as the car was in some state.
Both cars had overtaken a bus which had stopped at a bus stop, the car in front had signalled to overtake the bus and left his indicator on because he intended to turn right about 20 yds further on, the driver of the rear car assumed the lead car was going straight on and had omitted to cancel his indicator; meanwhile, the driver of the car behind was distracted by his front seat passenger having difficulty fastening her seatbelt and doesn't see the lead car stop to allow oncoming traffic to clear, in that instant the collision happened.
The correct thing to have done was to make sure all passengers have their seatbelts on before driving off, then the accident wouldn't have happened.
Aye. It's actually a offence to not wear one. My partner had a situation rise up on that but it was involving someone under 14 years old. He had forgotten/didn't know that under 14 it is his repsonsiblity as the driver to make sure the person was wearing a seatbelt. Myself personally i'd refuse to drive untill the person had put it on but then i'm like that lol.
oldmarine
31-Mar-11, 00:05
I thought this was going to be a joke until i read the last line.
No joke here. I wish my 85 year-old engineering mind could calculate this one for joxville. Looks like an honest question.
In regards to the seatbelt wearing, I have actually stopped the car, got out and smacked the kids hands for taking their arms out the seatbelts in their carseats after telling them, then screaming at them, counting to 3 etc.....
Its something that I am trying to drum into them..clunk click every trip.
But our car beeps at you if someone has not got their seatbelt on, and I refuse to move, or let them walk...(depending on age/distance to go etc) but the car doesnt move until seatbelts are on!
I would do the exact same Dadie lol. Its so important that children learn that they must wear a seatbelt and in the correct way even when they don't like it.
In my opinion to allow your child to grow up thinking its ok to not wear a seatbelt or to let them wear one the wrong way is irresponsbile. May seem harsh to some but at the end of the day the seatbelt when worn correctly will save a life. You never know when you will rely on the seatbelt to make the difference between life and death.
Not quite there, but sobering thoughts:
It’s surprisingly easy to generate 30 Gs: a vehicle coming to a stop in four feet from 60 mph would generate a 30-G deceleration.
Imagine the outcome when an unrestrained 10-pound baby slams headfirst into the dashboard in a 30-G collision, concentrating 21 stones of force on its skull and spinal column.
High G loads are common even at low speeds, which explains why child safety seats are so vital. For example, the dummies in the federally mandated 30 mph crash test routinely register 40 to 50 Gs. Even though a restrained passenger would probably survive the collision, a 10-pound baby would weigh 28 to 35 stones. Could someone be expected to hold onto a baby in such a crash? The answer is no.
http://www.vcu.edu/cppweb/tstc/crashinvestigation/kinetic.html
orkneycadian
31-Mar-11, 13:39
The question is, what would the rear seat passenger's equivalent weight go up to due to G-force involved?
Far too many variables. G Force in this context is generated by rate of deceleration - how fast something slows down. Factors affecting rate of slowdown of the rear seat passengers include.....
How far did the car in front roll forward when hit, as opposed to being completely solid (like a immovable concrete wall for example)
How much did the rear crumple zone on the car in front collapse
How much did the crumple zone on the front of the moving car collapse
Was it a straight head on (well, head to tail on) or a glancing blow
What other things slowed down the passengers - On a 1 year old car, we can assume airbags were invovled?
Which bodily parts of the 2 passengers collided, and how they resilient or pliable they are. For example, if the rear passengers apparantly ample sized belly contacted first, then it would give a fair bit, dramatically reducing the G force, compared to if it were a more solid bodily part
How much of the rear passengers weight was behind the point of impact and contributed to the impact.
Sorry of that sounds "boffiny", but there are a lot of variables, and changing them by a little bit makes a big difference. For example, if a car were designed with no crumple zone, and in impact, came to a stop with the passenger cell moving just 6", the occupants would sustain twice as many G's as those in a car with a modest crumple zone that brings the passenger cell to rest in 12". Thats only 6" of crumple zone, but it halves the G forces in a simple comparison.
The thing is, in relation to the G force question, is that there are so many variables that could make a significant difference to the G force from only a minor apparent change in circumstances.
Like in school science experiments where it was easier to "ignore heat loss / friction / drag" whilst carrying out experiments, its much simpler to calculate G forces when something solid hits something solid and slows down in x inches. As soon as something of undefined solidity hits something else of undefined solidity in an undefined manner, the outcome could range from one end of the spectrum of possibility to the other!
Sorry that thats boffiny and vague, but thats science for you!
caithness-chicky
31-Mar-11, 17:57
Orkneycadian is right that there are many variables to say for certain but here's my take on the numbers it might give you a rough idea (I am by no means a boffin but do know a little physics, but don't quote me on that!):-
By estimating that the backseat passenger travels 0.5m before hitting the back of the front seat
25mph = 11.18m/s²
20 stones = 127Kg
Deceleration = (Initial Speed² - Final Speed²) / (2 x Distance) = (11.18² - 0²) / (2 x 0.5)= 125m/s²
Newton’s 2nd Law of Motion – Force = Mass x Acceleration (or deceleration in this case)
F = 127 x 125 = 15875 Newtons / 9.81 (gravity) = 1618Kg = 1.62 Tonnes
So this gives you the force that the back seat passenger is exerting on the front seat, but if the frontseat passenger is not wearing a seat belt (?) then I would have thought that they would also be propelled forward out of their seat at the same time as the back seat passenger, and therefore would be hitting the window under the force of their own weight and nothing to do with the back seat passenger? So if your friend weighs for example 10 stones and the distance they travel from the front seat to hitting the windscreen is still 0.5m then the force is half that of the 20 stone passenger ie. 0.81 Tonnes (810Kg)
someone please correct me if I'm wrong
orkneycadian
31-Mar-11, 18:16
Surely the passengers would only start slowing down when they hit something? - the seat in the case of the rear passenger, the windscreen in the case of the front? The 0.5m they would travel between normal seating position and contact with whatever is about to slow them down and make them say "ouch", would be covered are near original speed, in this case 25 mph?
They would only really start slowing down after making contact with something, and the distance in which they slowed to rest would be governed by the object which was slowing then down?
A good reason not to have loose stuff in the car above the back seats in the car!
No need to have missiles from the back hitting you too.
George Brims
31-Mar-11, 22:26
A good reason not to have loose stuff in the car above the back seats in the car!
No need to have missiles from the back hitting you too.
I wrote a short story/essay in high school about a man stealing his wife's wee yappy dog after she kicked him out of the house. He then died from being hit in the back of the head by the dog when he had a collision. I got good marks and the comment "Rather sick".
The question is, what would the rear seat passenger's equivalent weight go up to due to G-force involved?
Technically the persons weight, and indeed mass would remain the same. As would their momentum. In order to calculate anything else really we would need more information.
A good reason not to have loose stuff in the car above the back seats in the car!
No need to have missiles from the back hitting you too.
Or heavy items in the boot of a hatch or estate. They will come through the seats.
~I know from experience....paint ...wee dog ..brakes...car!
expletives!
Sore arm and painted interior of said car.
not fast speeds..less than 25 mph...
Well about 20 years ago, my sister and brother-in-law were up visiting, and we had just been to Thurso and purchased a couple of pounds of pork chops (bone in) which he placed on the back shelf.
We were driving along the straight by Dounreay, when suddenly 'Roy' jammed his anchors on, as he had missed a turning he intended to go up. The chops nearly took his head off and landed on the dash-board - it was ever so funny!
Well about 20 years ago, my sister and brother-in-law were up visiting, and we had just been to Thurso and purchased a couple of pounds of pork chops (bone in) which he placed on the back shelf.
We were driving along the straight by Dounreay, when suddenly 'Roy' jammed his anchors on, as he had missed a turning he intended to go up. The chops nearly took his head off and landed on the dash-board - it was ever so funny!
Oh, how we laughed all the way to A&E
oldmarine
01-Apr-11, 22:37
George, some people just don't understand your sense of humor. lol!!!
Oh, how we laughed all the way to A&E
Uh? "The chops nearly took his head off and landed on the dash-board"... (i.e. 'nearly' as in 'missed'...
orkneycadian
02-Apr-11, 10:22
Or heavy items in the boot of a hatch or estate. They will come through the seats.
Another item for the "how do we cut down on road deaths" thread then! Boy racers with 10 megawatt amplifiers and huge woofers (and no, I dont mean Great Danes) in the boot! [lol]
~I know from experience....paint ...wee dog ..brakes...car!
expletives!
Sore arm and painted interior of said car.
not fast speeds..less than 25 mph...
It could have been worse, check these pics: http://www.bing.com/images/search?q=paint+car+accident&view=detail&id=282B9079B8913A0F575C6F3B85B6D71B027F40C9&first=1&FORM=IDFRIR
orkneycadian
02-Apr-11, 10:25
That is Dadie isn't it?
Technically the persons weight, and indeed mass would remain the same. As would their momentum. In order to calculate anything else really we would need more information.
Sorry, I can't provide more info.
Thanks for all replies to my question.
orkneycadian
02-Apr-11, 10:26
That is Dadie isn't it?
No, sorry, my mistake - Thats just a dude with long hair....
No the paint was willow green!
I was on my own in the car...it was my hubbys car ....not my wee car....
Think I might be a wee bit slimmer than the guy in that car!
But I was told to hit the dog next time!
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