I need a question Answered, Invovles Maths, Physics and cars.

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Yestday morning my girlfriends car got plowed into the back of while it was parked on the side of the road.
The girl that hit it is alright altough in a fair bit of shock which is the main thing thing but Both cars are written off.

Im just curious to how fast the other driver was going.

The Girl driving was coming home from night shift and feel asleep and went straight into the arse of the Falcon with out braking at all.
The impact was hard enough that both cars got stuck together and needed 2 trucks and a forklift to pull them apart.

So i have gatherd these small bits of info and hope someone can work out how fast she was travelling. Because i dont think it was as fast as some people may think(I think this because she never got a chance to brake)


The impact of the cars moved the falcon 20 Meters up the road from where it was parked(Still with the celica stuck in its boot.)

The Falcons weight is 1540kgs

The Celicas(ST184 from what i could tell) weight 1285kg

Also the Falcon is an Auto and was in park and had the hand brake on.

It looked like it was an 80-20 impact, So fairly square on, Just off to one side a bit

So from this how fast do you think the Celica hit the falcon?

Thanks:)
 
Can you do this without knowing the time it took for the car to move the 20m? My physics is old and rusty but I am pretty sure you need to know that to work it out.
 
Assuming your wheels being locked would stop you on a 30 degree slope gives a coefficient of dynamic friction of tan 30 = 0.6. If the other car was not breaking at all that gives a stopping force of 9000N. Work = force x distance so the energy just after the collision would be 9000 x 20 = 180000. Energy = 1/2 mass x velocity squared.

m = 1540 + 1285 = 2800 kg

> velocity = 10 m/s = 20 mph (accurate to about +40/-10 mph)
 
namnoc said:
Assuming your wheels being locked would stop you on a 30 degree slope gives a coefficient of dynamic friction of tan 30 = 0.6. If the other car was not breaking at all that gives a stopping force of 9000N. Work = force x distance so the energy just after the collision would be 9000 x 20 = 180000. Energy = 1/2 mass x velocity squared.

m = 1540 + 1285 = 2800 kg

> velocity = 10 m/s = 20 mph (accurate to about +40/-10 mph)
Click to expand...

LOL so between 10mph and 60mph :/

dP
 
This is an incredibly difficult question!

Problem 1: If we were talking about two objects colliding in a perfectly elastic manner, ie. kinetic energy being conserved, this would a pretty easy calculation. But the plastic deformation of all that crunching metal means that we have no idea how much kinetic energy was conserved during the collision.

Problem 2: We don't know the specifics of how the cars moved the moment after the collision occured. Yes, they moved 20m, but to even guess an initial speed we would need the time to travel 20m.

So in short not enough information to accurately work out what speed the girl was going.

So, time to guess! The highway code says that it takes an average car 14m to stop dead from 30mph using the brakes. In your case the energy was removed from the car by the deformation of the metal in both cars, as well as the handbrake in the front car. I'd be surprised if the handbrake would be up to much in a rear shunt, so to me 20m doesn't sound unreasonable for a 30-40 mph collision.
 
A quick search suggests a skidding car slows at around 0.7g, probably less for two cars wrapped together so let's guess half that at 0.35g.

v^2=u^2+2as, and putting v=0, a=-3.5, s=20 in gives u = 12ms-1

This is the speed that both cars start moving together at after colliding. Using conservation of momentum and ignoring energy absorbed by cars crumpling, etc.

mass of celica x impact speed = mass of both together x u
so impact speed = (1540+1285) x 12 / 1285 = 26ms-1
gives impact speed of 59 mph

All based on massive guesswork and approximations. Good job your girlfriend wasn't in it at the time!
 
So what was the speed limit in the area? We can probably assume the girl was doing at most +10 above the limit. Would be interesting to see who came closest :p
 
you would also need to know the coefficient of friction between your tyres (at least the ones held by the handbrake) and the road plus the same for the handbrake pads as LOTS of energy will have been converted to heat there.

Anyway, who cares about her speed? No 3rd party injury or witness so plod will have zero interest in doing her.

Put your time and effort instead into collating lots of written evidence to support a proper write-off value for your car - the first offer you receive from her insurer will be crap (and, unless you have add-on legal protection cover, your insurer will not give a toss)!
 
Zip said:
Y

The Girl driving was coming home from night shift and feel asleep and went straight into the arse of the Falcon with out braking at all.
The impact was hard enough that both cars got stuck together and needed 2 trucks and a forklift to pull them apart.
Click to expand...


I know this is no answer to your question but I think someone has to be pretty special to fall asleep whilst cognitively in control of something.

People like this should be banned from driving if this is indeed the case.
I can't fall asleep unless I try to, I don't decide whilst driving in my car, yeah..might just close my eyes, sit back and nodoff.

If I'm THAT Tired I still can't just fall alseep, even after days of staying up, I can't just nod off I'm fully aware of what happens.

Also add the fact if I close my eyes I can't see where I'm going, I don't want to crash and I don't have a deathwish.

She must be mentally retarded.

Makes me angry! :(
 
ChrisJSY said:
I know this is no answer to your question but I think someone has to be pretty special to fall asleep whilst cognitively in control of something.

People like this should be banned from driving if this is indeed the case.
I can't fall asleep unless I try to, I don't decide whilst driving in my car, yeah..might just close my eyes, sit back and nodoff.

If I'm THAT Tired I still can't just fall alseep, even after days of staying up, I can't just nod off I'm fully aware of what happens.

Also add the fact if I close my eyes I can't see where I'm going, I don't want to crash and I don't have a deathwish.

She must be mentally retarded.

Makes me angry! :(
Click to expand...

For most people, falling asleep when you're tired isn't a choice. Yes, it's possible to force yourself to stay awake while driving, but the difficult part isn't falling asleep, it's staying awake and concentrating. Of course, the amount of extra concentration required to keep yourself awake when totally exhausted just makes the situation worse.

I'm not condoning driving when you're so tired that you can just pass out, but what you said is really ridiculous.
 
Vixen said:
I'm not condoning driving when you're so tired that you can just pass out, but what you said is really ridiculous.
Click to expand...

EDIT: Nobody I know have this problem of not having a choice and falling asleep when extremly tired.
I've have this conversation before so I know this with them at least.

If you have a medical condition, then what the hell are you doing driving whilst tired when you know you don't have a choice staying concious .

I might sound ridiculous, but I don't have any kind words for what you sound like.

Like I said, if you have problems falling asleep whilst in command of a 1000kg + vehicle driving at speeds in excess of a human running, you have to be VERY SPECIAL in the head.


PS. Had to edit this so many times to make any sort of sense.
 
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ChrisJSY said:
I know this is no answer to your question but I think someone has to be pretty special to fall asleep whilst cognitively in control of something.

People like this should be banned from driving if this is indeed the case.
I can't fall asleep unless I try to, I don't decide whilst driving in my car, yeah..might just close my eyes, sit back and nodoff.

If I'm THAT Tired I still can't just fall alseep, even after days of staying up, I can't just nod off I'm fully aware of what happens.

Also add the fact if I close my eyes I can't see where I'm going, I don't want to crash and I don't have a deathwish.

She must be mentally retarded.

Makes me angry! :(
Click to expand...


Hmmm, it is a very serious mistake to make and people could have died but thankfully no one did. It was an accident at the end of the day, unless you have worked night shifts you wont understand. Perhaps you stay up late some nights but it is different to do it all the time.

If she had hit someone it would be manslaughter not murder I suspect.
 
Not possible to make an assesment based only on projectile-type facts, unfortunately. The key factor is how much energy was absorbed by the crumpling of the two cars, which will be significant (since, as you say, they were stuck together tightly enough that a forkift was required to separate them).

The insurance company will have experts to make such assesments. They will have calibrated computer models, and will assess the damage to the car (along with the facts from the road) to obtain a set of parameters. From this they will be able to estimate the speed of impact. Actually, since both cars were a write-off, they probably won't bother. But still - they would have the neccesary tools and expertese, should the data be required for criminal prosecution purposes.
 
starfighter said:
unless you have worked night shifts you wont understand.
Click to expand...

wow.

Yes I have worked night shifts, but even if I did not, are you telling me people don't get tired from working in the day either? or those who have trouble sleeping?

Won't understand, hah.

I understand that if you know you have these problems you shouldn't be on the road, it's that person who didn't understand the concenquences of her ability to stay awake and the fact she did a night shift.
 
ChrisJSY said:
wow.

Yes I have worked night shifts, but even if I did not, are you telling me people don't get tired from working in the day either? or those who have trouble sleeping?

Won't understand, hah.

I understand that if you know you have these problems you shouldn't be on the road, it's that person who didn't understand the concenquences of her ability to stay awake and the fact she did a night shift.
Click to expand...


wow :rolleyes:

yes people get tired during the day obviously, unsure who said they didn't and who cares, you have worked nights and so has the person who crashed.

It is pretty much fact that working nights makes you generally tired, especially on the way home where she might have been up 24 hours.

This does not excuse her but it means she didn't do it on purpose. we are not machines. do you think when she left work she had an error message saying "you will fall asleep in exactly 13 minutes". knowing that her drive would be longer she is obviously a criminal.

no matter how tired I am after a night shift I am driving home. you can't exactly sleep on the office floor. perhaps in a perfect world it would be nice to have a nap but when your own bed is minutes away...
 
Driving on the verge of falling asleep is as bad as drink driving.
Give me a break, you're defending a stupid reason.
 
ChrisJSY said:
Driving on the verge of falling asleep is as bad as drink driving.
Give me a break, you're defending a stupid reason.
Click to expand...

I don't think anyone is claiming that driving when you're that tired is acceptable, more commenting on your claims that nobody "just falls asleep" when extremely tired.

I know if I'm sitting down when I'm very tired I will nod off without trying but then again I know not to put myself in control of a vehicle when I am in such a state.
 
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