# Accident Reconstruction, Vomhof equation?

1. Oct 28, 2011

### turnwrite

Hey Physics Forums, I am currently trying to wrap my brain around the physics of accident reconstruction. Specifically, determining the velocity of a vehicle that collided laterally with one or more "narrow" objects.

Here is the info I have on the subject:

[PLAIN]http://img231.imageshack.us/img231/8589/screenshot20111027at103.png [Broken]

Also, this PDF explains it quite a bit more technically, if you scroll down to its discussion of the "Vomhof equation."

http://www.4n6xprt.com/Equation_Comparison-Frontal_Narrow_Object_Impacts.pdf" [Broken]

So with that, here are the two things that I don't understand!

1. Why is it that, when a car hits a "narrow" object (such as a utility pole), the equation for determining its velocity from the depth of the crushed area on the car must be adjusted? Both links multiply the equation for velocity by .6 when the vehicle strikes a narrow object. But if the size of the depressed area on a vehicle post-impact is proportional in any way to the speed it was going pre-impact, shouldn't that proportion remain the same regardless of whether the car hits a wall or a dumpster or something narrow, like a pole? Why the .6 adjustment?

2. Given that there is some reason to distinguish between a car crashing into a narrow object and a car crashing into any other object (as described in that first question), how is "narrow" defined? Both of the above links give examples of "narrow" objects, but no strict definition! So how can a reconstructionist possibly know whether to solve for velocity using the generic equation or the "narrow" object one?

Any help and insight would be greatly, greatly appreciated! ESPECIALLY on the second question!

Last edited by a moderator: May 5, 2017
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