For all you physics gurus...help me

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Posted by torontotrance on Jul-02-2004 03:58:

For all you physics gurus...help me

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.

Posted by Zenchowdah on Jul-02-2004 04:03:

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

Posted by trancebrat on Jul-02-2004 04:05:

Re: For all you physics gurus...help me

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.

Posted by torontotrance on Jul-02-2004 04:14:

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.

Posted by trancebrat on Jul-02-2004 04:15:

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.

Posted by Zenchowdah on Jul-02-2004 04:17:

Posted by cbxzcm on Jul-02-2004 04:19:

Re: For all you physics gurus...help me

Posted by torontotrance on Jul-02-2004 04:22:

fucking complicated then

Posted by Zenchowdah on Jul-02-2004 04:25:

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.

Posted by Flyboy217 on Jul-02-2004 04:30:

Re: For all you physics gurus...help me

Posted by torontotrance on Jul-02-2004 04:31:

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

Posted by Zenchowdah on Jul-02-2004 04:32:

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?

Posted by Zenchowdah on Jul-02-2004 04:34:

Re: Re: For all you physics gurus...help me

quote:
Originally posted by Flyboy217
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.

oo, yeah, listen to him. hes smart

Posted by Flyboy217 on Jul-02-2004 04:34:

quote:
Originally posted by torontotrance
so 10.1m/s = how many km per hour?

Not 10.1 m/s. 10.1 m/s^2, an acceleration. This is a reasonable rate for a car to decelerate. Nothing else (momentum, mass, etc.) needs to be considered.

*edit* Consider it this way. From a standstill, if you were to accelerate at 10.1 m/s^2, it would take you 2.65 seconds to get to 60 mph. That's very quick acceleration, but not neck-snapping. If he was hard on his brakes, it's feasible that he could do the reverse (40 mph to 0 using that deceleration). Double-check me though. Cross reference deceleration rates on the internet.

Posted by Flyboy217 on Jul-02-2004 04:44:

quote:
Originally posted by Flyboy217
Not 10.1 m/s. 10.1 m/s^2, an acceleration. This is a reasonable rate for a car to decelerate. Nothing else (momentum, mass, etc.) needs to be considered.

*edit* Consider it this way. From a standstill, if you were to accelerate at 10.1 m/s^2, it would take you 2.65 seconds to get to 60 mph. That's very quick acceleration, but not neck-snapping. If he was hard on his brakes, it's feasible that he could do the reverse (40 mph to 0 using that deceleration). Double-check me though. Cross reference deceleration rates on the internet.

http://www.skytran.net/09Safety/10sfty.htm

As you can see, up to 60 g's is considered "safe." We're dealing with ONE g. From 30 mph to 0, they list 1 foot as a safe stopping distance (sounds absurd to me, but hey). Mind you, this says nothing about the car (it would probably become junk at this rate), but a human wouldn't feel much at 1g.

Posted by N][ck on Jul-02-2004 04:48:

Re: For all you physics gurus...help me

quote:
Originally posted by torontotrance
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.

Ok, d = v*t + (1/2)*a*t^2 is your basic physics formula we're gonna use here.

The only problem is, we don't know how long it took for him to lay the 45ft of skid marks. So we'll swap crap around to get rid of the time element in the second part of that equation.

a = v/t so t = v/a

swap that into the second bit and we get:

d = v*t + (1/2)*a*(v/a)^2

stuff cancels and we are left with:

d = v*t + (v^2)/(2a)

so, in this equation:
d = braking distance
v = the speed the guy was going before he started braking
t = the idiot's reaction time
a = rate of decelleration

if we give this guy the benefit of the doubt, and say he has some kickass reaction time, t = 0.3 seconds
he says he was going 60km/h --> 17m/s approximately
and lets say his tires are ok and he has decent brakes, so he's decellerating at 1g = 9.81m/s^2
(It is unlikely that he was slowing down at this rate, since his tires were sliding on the asphalt).

So, plug that stuff in and you get a theoretical braking distance (from 60km/h to 0km/h), if he was a kickass driver and didn't lock up the brakes of: 20 meters. --> a little over 60 feet.

if you eliminate reaction time from the whole thing, and just look at from the point where his tires locked up to where he hit the car, then the distance is 45ft.

So, since the car didn't end up at 0km/h, millimeters away from the other car, after a 45ft brake lockup, you can be pretty confident that he was going quicker than 60km/h when he locked up the tires.

Sorry for the long post...time to watch Ali G.

Posted by Flyboy217 on Jul-02-2004 04:53:

quote:
Originally posted by Flyboy217
http://www.skytran.net/09Safety/10sfty.htm

As you can see, up to 60 g's is considered "safe." We're dealing with ONE g. From 30 mph to 0, they list 1 foot as a safe stopping distance (sounds absurd to me, but hey). Mind you, this says nothing about the car (it would probably become junk at this rate), but a human wouldn't feel much at 1g.

Better yet, consider this:

http://www.skytran.net/09Safety/02sfty.htm

1g is apparently a good limit for a clean dry road with very good tires. If you factor in reaction time (as N][ick did), then it does start to seem less likely. It's close, but now I'll go with "no" for the guy.

Posted by Photo_bot_2k1 on Jul-02-2004 04:56:

if we knew the type of car he had a quick check to www.kbb.com could tell us the weight
and therefore we can calculate the coefficient of friction on the ground (becuase he slid)

compare it to the normal friction coefficient of the ground and see if its way off

also you could probably tell how fast he was going when he started skidding

Posted by Flyboy217 on Jul-02-2004 04:59:

quote:
Originally posted by Photo_bot_2k1
if we knew the type of car he had a quick check to www.kbb.com could tell us the weight
and therefore we can calculate the coefficient of friction on the ground (becuase he slid)

compare it to the normal friction coefficient of the ground and see if its way off

also you could probably tell how fast he was going when he started skidding

The coefficient of kinetic friction between rubber and asphalt is a constant. The deceleration figures are sufficient to describe the braking of a car.

Posted by blazed it on Jul-02-2004 05:51:

this is all gibberish to me.

Posted by Photo_bot_2k1 on Jul-02-2004 05:53:

quote:
Originally posted by Flyboy217
The coefficient of kinetic friction between rubber and asphalt is a constant. The deceleration figures are sufficient to describe the braking of a car.

yeah of course thier constant thats why you compare the value you get from your calculations to the book values for the asphault vs rubber

Posted by jonze on Jul-02-2004 05:58:

doesnt the insurance company have a way to figure it out? i know when i got into an accident the cop figured out that the kid was speeding and issued him a ticket on the spot. he started crying and i started laughing (it was one of those evil villian laughs like in cartoons). in the end it doesnt matter because the insurance companies get together and find the best way to screw over their clients.

hope your mom is alright.

Posted by PhloTron on Jul-02-2004 06:00:

http://www.e-z.net/~ts/speedch.htm