The Traffic Accident Reconstruction Origin -ARnews-

Re: Pedestrian Accident Reconstruction

Ed Phillips (edphill@aol.com)
Sat, 15 Nov 1997 01:58:49 -0500 (EST)

Joe,

I'm not pissed off at anybody. You don't know me well enough to spot my droll sense of humor. (Though some have spelled that "drool" and others "dull") Nonetheless, please point out where Fricke says a car traveling in a certain direction, say north will impart an opposite force to an object inducing a southerly motion. I don't read that in his chapter nor see it implied. If you are suggesting exhibit 20 of 877, that is a special case where the body is severed and obtains little of the vehicle velocity.

The vehicle has a comparatively LARGE momentum to the north. The pedestrian has no momentum north (standing still). When struck; Newton 3 tells us the forces imparted are equal and opposite. In a full contact a 96.6 lb person struck by a 3220 lb car (for illustration) receives 33.3 times the speed change of the car. So they get sped up a lot, but the car gets slowed down only a little (0.03 of the ped's increase). And the ped would be pushed away in front of the car.

In this subject type of event, with other than full contact, the ped gets accelerated some portion of that 33.3 and the car slows some portion of that 0.03. So the car pretty much continues forward at the same speed. The ped is not fully engaged so they don't get the same speed as the car, just a portion of the northerly momentum. The car is traveling north at 35 mph, the ped is accelerated northerly to (pulling a number from the air) 22 mph.

Now if you and I are running side by side and I instantly go to 35 mph and you instantly go to 22 mph, I pass you. That's what is happening. You're not traveling south, you're traveling north less fast. There is NO southerly component, because there is no applied southerly force.

I have seen many auto/ped's and I've never seen a body do what you describe. If it does it's violating the Laws of Physics and as such needs special study.

Ed


Ed Phillips
edphill@aol.com

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