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In accident reconstruction, a relationship frequently used for completely inelastic collisions is one between relative speed (or closing speed), vehicle weights and total energy dissipated in deforming the vehicles during collision (crush energy). Derivation of this equation is presented in various accident reconstruction literature, but always for collinear collisions, i.e. vehicles traveling along the same straight line. A derivation is presented here for the general case, which shows the same relationship is valid for any angle of approach between the vehicles, not just 0° or 180°. A supplemental derivation is presented which yields the relationship for all collisions, not just completely inelastic collisions.

Several accelerometers are now commercially available and are becoming widely used by accident reconstructionists for determining the coefficient of friction or the “drag factor” using an exemplar vehicle at the accident site. To get a quick estimate of the effective coefficient of friction for the braking test, people often simply average the deceleration values in units of g (gravitational acceleration) from the accelerometer output of deceleration versus time. Since this involves an inherent error in method, it is of interest to examine the magnitude of this error.