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Old 06-20-2007, 04:54 PM   #1
DataDan
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I ran across an article today that I found interesting in a tragic sort of way...

Last week, in a bike-vs.-bike crash that occurred in a large group of sportbikes, a young woman was killed when she was thrown from a bike, slid into oncoming traffic, and was hit by a car. The police want to know how fast the bike was going at the time of the crash, but the only hard fact they have available is the distance the woman slid along the pavement. To find initial speed, they must determine the coefficient of friction of the sliding victim.

So they set up an experiment with a deputy's daughter, recruited for size similar to the victim's, dressed in the same gear as the victim (see attached pic).

The MAT team tried to re-create the accident's conditions as closely as possible. Investigators bought Megan a leather jacket, jeans and boots that matched what Enriquez was wearing. They also conducted their simulation early in the morning to match the temperature at the time of the accident as much as possible.


Investigators had Megan put on a harness. She then lay on her back in the middle of 700 East in a spot where paint that marked the accident location from a week earlier was still present. Several sheriff squad cars with their red and blues lights flashing surrounded the young girl and investigators to prevent them from being hit by morning commuters.
Hooking a special scale to the harness, Anderson gave the device a couple of short tugs, dragging Megan just an inch or two across the pavement.
Anderson then recorded the information on the scale, and in less than two minutes the simulation was over.
Deputies will now take that information back to their office and calculate the minimum speed of the victim when she crossed the road as well as the speed of the motorcycle to determine if any criminal activity was involved.
From the Deseret Morning News, Salt Lake City.
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Old 06-20-2007, 05:43 PM   #2
mugwump58
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From my limited recollection the way they are measuring they are only going to come up with the initial force required to over come the still state which will be much higher than the force required to keep the object moving. I think they are going to come up with a much higher velocity than they should. I'm sure that one of the many engineers here will fill us in on the vaguaries of my recollection.
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Old 06-20-2007, 07:19 PM   #3
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There are engineers here? Who? Where?

The experiment could get useful results, but you're right that they could be measuring the (higher) static coefficient of friction rather than the dynamic coefficient. It basically depends on whether they read the force guage when she was in motion or simply recorded the highest value. You're also right that if they use the higher number, they'll end up with a higher speed.

Most acccident investigators are pretty intelligent - they'll know at the very least basic physics, and probably a lot more. I'd give them the benefit of the doubt.
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Old 06-21-2007, 07:27 AM   #4
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One problem is that the test doesn't take into account whether or not the victim was sliding or rolling on the ground. Or a combination of sliding and rolling. Seems a pretty pointless experiment that any decent defense attorney could torpedo with ease.
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Old 06-21-2007, 08:04 AM   #5
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Excellent poin seruzawa (what the heck does that mean, anyway?) I was about to make it myself. Trying to relate a human body tumbling after a crash to a car sliding down the road with the brakes locked (which I'm sure is their premise) is dumb.

Further, what's the point of all this? She's dead, this exercise won't help pin the blame on anyone but her, there must be something better they can do with their time.
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