The Traffic Accident Reconstruction Origin -ARnews-

Re: Chain reaction accident

Brian P. Griffin ([email protected])
Sat, 19 Dec 1998 12:48:01 -0500 (EST)

Sounds like there may be a couple of issues:

Paul R writes:
>I've been square rooting ke*64.4 and dividing it by the weight of the
>striking vehicle and then square rooting that for velocity

First off, it sounds like one square-root too many. Take Ke, multiply by
64.4, divide by weight, THEN take the square-root (only once!) to get velocity.
The relation is:

v^2 = 2*g*Ke/W

The extra square-root you seem to be taking will definitely make your
final number too small!

I'm guessing that you're getting kinetic energy by adding up the work done by
the vehicles while rolling to a stop (W*f*d). Unfortunately, this ignores all
damage, and will therefore underestimate the collision speeds.

A better approach would be to calculate each vehicle's post-impact speed using
the travel distance and an appropriate drag-factor, then use
conservation-of-momentum to calculate the pre-impact speeds. If the vehicles
really were stopped initially, then it sounds like you have all the info
you need. Now, you just have to come up with appropriate drag-factor(s), and
you're home free!

Brian P. Griffin
[email protected]

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