## 151 General

A milestone in the development of modern science was the publication of Newton's three laws of motion. In brief, the three laws are as follows:

I. A body at rest tends to stay at rest and a body in motion tends to stay in motion unless acted upon by an external force.

II. When a force is applied to a free body, the rate at which the momentum changes is proportional to the amount of force applied. The direction in the change of momentum caused by the force is that of the line of action of the force.

III. For every action by a force, there is an equal but opposite reaction.

The above three principles form the basis for all nonrelativistic mechanics. Until vehicle velocities begin to reach about 3000 km/s, or about 1% the speed of light, the above three basic axioms of motion will do for every instance of vehicular accident analysis. Since the present relative velocities of impacting vehicles rarely exceed 200 km/hr or 0.0000185% the speed of light, there is little need of having to apply Lorentz transformations or other rela-tivistic considerations to vehicular accident analysis.

Of course, this brings to mind the old joke about the pedantic motorist who told the judge that he did not run a red light; he was traveling fast enough that the red light appeared green to him. The judge, no stranger to

Doppler effect calculations himself, simply agreed with the motorist and charged him \$1 for every mile per hour over the speed limit.

In reviewing Newton's three laws, it is apparent that the analysis of momentum changes and forces is central to their application. In vehicular accidents, impact with a second vehicle usually causes changes in the direction and speed of both the involved vehicles. The analysis of the momentum and forces in such cases can provide valuable information about the accident, especially pre-impact speeds.

Sometimes when the available accident data are insufficient for other types of analyses to work, there is sufficient information to apply momentum methods to analyze the accident. Also, the application of momentum methods is an independent way of corroborating results from other types of analyses.

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