The Traffic Accident Reconstruction Origin -ARnews-

Re: Chain reaction accident

Ed Phillips (edphill@aol)
Sat, 19 Dec 1998 17:55:34 -0500 (EST)

Paul,

Each answer will come up short because not all energy is being considered. In the IPTM example, the situation is set up to say that all KE is coming from the bullet vehicle (m/c). And as energy in conserved, all energy after impact with the stopped vehicle (zero speed equals zero motion energy at impact) must have been introduced by the m/c.

While this is true, not ALL post-impact energy is accounted for in the set-up, therefore all results will be minimums.

KE(before) = KE(after)

KEb = [W(m/c)*v^2] / 2*g

KEa = KE1(to move car to POR) + KE2 (to move m/c to POR) + KE3(damage to m/c) + KE4(damage to car) + KE5(heat energy losses, acoustic energy losses etc.)

KE5 is typically negligible.

But as you can see if you are not including KE3 and KE4 you are minimizing the post energy which in turn minimizes the pre-energy.

KE = 0.5*m*v^2 or m*a*d

To get reasonable results all post-collision energy must be quantified if possible. If the components can not be quantified it's better to try momentum or some other approach.

Ed

Ed Phillips
edphill@aol

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