Someone I know got into an accident and the speed of the guy who hit the person that I know is in question

so if the skid marks say it took him 45ft to stop and he says he was going 60kph, is there anyway to figure it out via some formula if the dude is telling the truth.

need to know the weight of the car, its a momentum problem.

momentum = mass * speed. erm... there is also a lot to do with friction between the tire and the road.. application of braking.. this is not my area. i failed physics

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quote:

Originally posted by torontotrance Someone I know got into an accident and the speed of the guy who hit the person that I know is in question so if the skid marks say it took him 45ft to stop and he says he was going 60kph, is there anyway to figure it out via some formula if the dude is telling the truth.

What kind of vehicle was it that hit him? If the vehicle has an airbag (whether it deployed or not) it is possible that it also has a module that records the speed of the vehicle at the time of impact...some even up to five seconds before. Obviously not all vehicles are equipped with this module and not all record if the airbag does not deploy. Advanced ones record speed, braking, seatbelt usage, etc. Someone should check into it. It may be a dead end, but it couldn't hurt to check it out.

No airbag went off, the idiot hit my poor mother in the left wheel well and proceeded to blame my mother, she turned out of a parking lot at 5kph and the coast was clear but the clown was going waaaaaayyyyyyyy too fast and if I ever get my hands on him, I'll kick his ass. One of you ******s should be able to tell me how fast the fucking asshole was going.

quote:

Originally posted by torontotrance No airbag went off, the idiot hit my poor mother in the left wheel well and proceeded to blame my mother, she turned out of a parking lot at 5kph and the coast was clear but the clown was going waaaaaayyyyyyyy too fast and if I ever get my hands on him, I'll kick his ass. One of you ******s should be able to tell me how fast the fucking asshole was going.

Airbag does NOT have to deploy.

quote:

Originally posted by torontotrance No airbag went off, the idiot hit my poor mother in the left wheel well and proceeded to blame my mother, she turned out of a parking lot at 5kph and the coast was clear but the clown was going waaaaaayyyyyyyy too fast and if I ever get my hands on him, I'll kick his ass. One of you ******s should be able to tell me how fast the fucking asshole was going.

oi, unfortunate situation. know anyone in a college physics class?

might be able to pull a favor and get em to enlist their professor, if they're cool like that

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quote:

Originally posted by torontotrance Someone I know got into an accident and the speed of the guy who hit the person that I know is in question so if the skid marks say it took him 45ft to stop and he says he was going 60kph, is there anyway to figure it out via some formula if the dude is telling the truth.

You could probably get a reasonable estimate using kinematics of the other guy's speed if you knew the mass of the car, the time it took to stop, and the coefficient of static friction of rubber on concrete.

fucking complicated then

OK, momentum = Mass * Speed, and the coefficient of friction of a car tire on asphalt is .72. anyone know how to use these number to determine what the mass of the car would have to be in order to stop in 45 ft? of course, you have to convert ft to m in order for this to work.

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quote:

Originally posted by torontotrance Someone I know got into an accident and the speed of the guy who hit the person that I know is in question so if the skid marks say it took him 45ft to stop and he says he was going 60kph, is there anyway to figure it out via some formula if the dude is telling the truth.

60 kph = 16.66 m/s
45 ft = 13.7 m

v^2 = 2ax

a = 10.1 m/s^2

That's approximately an acceleration of -1g. It's very reasonable to believe, as quick cars can achieve a positive acceleration of 1g.

so 10.1m/s = how many km per hour?

quote:

Originally posted by torontotrance fucking complicated then

not too complicated.. had i not slept in class, i could figure this out easy i just need someone to give me how distance fits in there........ wait a minnit...


speed = d/t and speed = 60kph

so

d/t = 60kph

and if the skid marks were 13.716 meters

13.716/x = 60

60x = 13.716

he was stopping for ~.2286 seconds, according to his story.

i dont think thats what we were trying to find, though.

what we want is the real one.. sooo we'd need to use speed in the momentum formulaaaaaaaaaaaa

p=m*v, with p taking the form of kg-m/s

we know v, or at least his v, and we can figure out p if we really try, i think. in order to get him in any sort of trouble the mass of his car would have to come out to some rediculously large or small number, im guessing small. any idea what the make of the car was?

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