accident and distance

# Thread: accident and distance

1. ## accident and distance

here is one for all you "CSI" ppl out there.
watched my buddy hit a deer this morning.
I was watching the "csi" ppl at work. measuring, pic taking, etc......
i am not too sure how they figure out how fast you were going??
usually you need 2 figure to figure out an answer....time/distance=speed.
they only have the distance... how do they figure that out???

btw...hes fine..helmet and leathers did their job, just a BAD concussion.
thx in advance for the help!

2.

3. simple physics. E=mc2.
with an estimate on mass (weight) and distance traveled from the point of impact, they can calculate energy (speed) of the impact, required to propel said mass said distance. make sense?

4. Where did the deer hit happen?

Hope your friend's noggin is OK.

DNA's explanation I would concur with. Add friction as well but they will probably mess that part up if he was wearing leathers and over estimate his speed: less friction with leathers so he will slide farther giving an estimate that he was going faster than he was, IMO.

5. Originally Posted by DNAspark99
simple physics. E=mc2.
with an estimate on mass (weight) and distance traveled from the point of impact, they can calculate energy (speed) of the impact, required to propel said mass said distance. make sense?

That's the right formula if the entire bike's mass is to be changed into energy... note you'll blow a sizable hunk out of the earth.

The whole thing is a Physics 11 problem. The starting formula in an idealized situation would be the kinetic energy formula E = 1/2 * m * v^2.

From there it's a simple matter of determining the coefficient of friction (determing that for a tumbling motorcycle is an estimate but the police have charts and formulas based of experimental evidence), weight of the object and distance of sliding/tumbling. Once you know how much energy was lost during the collision then you know that there must've been that much energy *before* the accident. That's the kinetic energy... speed is an easy calculation from energy and mass.

^ that's simplified... I refuse to break out a textbook to be more exact but any high school student can solve the problem once the tricky bit is determined (coefficient of friction).

Most accident investigators solve this with a minimal grasp on physics but a solid understanding of their course work. I'll shred the hell out of the first officer that uses this kind of calculation to determine my speed in court.

6. yea it's not literally E=mc2.
but don't ask me for the specifics - I flunked every bit of maths since grade 4.

7. Originally Posted by heisenberg
Where did the deer hit happen?

Hope your friend's noggin is OK.

DNA's explanation I would concur with. Add friction as well but they will probably mess that part up if he was wearing leathers and over estimate his speed: less friction with leathers so he will slide farther giving an estimate that he was going faster than he was, IMO.
buddys ok...just asked the same few questions over and over again for hours. went to the hospital and x-rays and a CT scan just to make sure!

it was this morning at about 7am near cypress exit.
deer just jumped from the median and right onto his headlight. NO reaction time. a little black skid about 2' long and nothing but the color of red leather and gray/red paint from the bike.

thx for the info. i guess it is a little hard to figure out with out all the variables that they measure and data they have to do it, but it was a question i was pondering...thx!

8. I watched the news this evening ....

... they're reporting that it was the same deer (not dear) that ran into a Rogers Video on Sunday and made a mess of the place trying to get out.

... what a way to go?!

9. I've gotta go way back on this one, but with skid marks I believe it's
the square root of 2.47 x distance +/- elevation x coefficient of drag....but I"m sure I'll be corrected.
An engineer will make calculations that include the final resting place of the motorcycle, speed from skid marks, "crush," or "intrusion," of the vehicle plus some other factors.
I've never heard an engineer advise the exact speed. Their evidence (at trial) usually goes, "based on....." and then, "the minimum speed is calculated to be...."
But I"m not an engineer, so my opinion is about as worthy as it gets.....for the internet.

10. Originally Posted by Bigga_Boy
i am not too sure how they figure out how fast you were going??
usually you need 2 figure to figure out an answer....time/distance=speed.
they only have the distance... how do they figure that out???
Originally Posted by michael
An engineer will make calculations that include the final resting place of the motorcycle, speed from skid marks, "crush," or "intrusion," of the vehicle plus some other factors.
I've never heard an engineer advise the exact speed. Their evidence (at trial) usually goes, "based on....." and then, "the minimum speed is calculated to be...."
There must be so many variables - for instance, frame sliders. I have seen bikes at the track go sailing by merrily on their frame sliders barely slowing down in a shower of sparks. I have also seen bikes without sliders come to a grinding halt pretty quickly. Any estimate based on how far a bike slid would be a very, very rough estimate without skid marks to measure and eye witnesses to confirm speed before impact.

11. The cops use the math to make their best guess and then give their best opinion.

Opinions are like assholes though,

Everyone has one.

You can easily find a hired gun that will put a more accurate number on the findings using the police estimates. Some of the speed numbers and the obvious errors in some police calculations are obviously out to lunch.

12. Any estimate based on how far a bike slid would be a very, very rough estimate without skid marks to measure and eye witnesses to confirm speed before impact.
Really? I've seen engineers do speed calculations, and have taken a 1 week RCMP collision reconstruction course (11 years ago), being one of 3 civilians to do so at the time.
When you talk about frame sliders you're talking about the CD, or co-efficient of drag, which is calculated. Also the CD of the different surfaces the vehicle/tire is sliding on. For instance, a bike slides across the asphalt, onto a grassy median, across a concrete drive and then comes to a halt. There are 3 separate calculations that combine to make a "minimum," speed, not an "exact," speed. The engineering, or math, will get you close, but never exactly, guaranteed, spot on, with speed calculations.

13. 24 Hours is reporting that the rider was operating without a license. If that's the case, speed at time of impact is probably the least of your friend's worries.

14. Originally Posted by adiabatic
That's the right formula if the entire bike's mass is to be changed into energy... note you'll blow a sizable hunk out of the earth.
Yeah I was gonna say... speed of light doesn't really have much to do with braking distance.

15. Originally Posted by Regular Joe
24 Hours is reporting that the rider was operating without a license. If that's the case, speed at time of impact is probably the least of your friend's worries.
it was a learners license and he did not have it on him. so that wont hold in court.(also it only 2 points and \$138.00 fine, not too worried!)

16. Originally Posted by Bigga_Boy
it was a learners license and he did not have it on him. so that wont hold in court.(also it only 2 points and \$138.00 fine, not too worried!)
Local news...accurate as usual.

#### Posting Permissions

• You may not post new threads
• You may not post replies
• You may not post attachments
• You may not edit your posts
•