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|Roadway Friction Accident
|With regard to accident reconstruction, experts will often refer to frictional relationships with terms
like Coefficient of Friction (μ) or Drag Factor (f). While the two terms are related, they are not interchangeable. By
definition, the coefficient of friction is the ratio of the tangential (parallel) force applied to an object sliding across a
surface to the normal (perpendicular) force.
In the English system, force is measured in pounds. For example, if a square block were set on a level surface, the
block would apply a measureable force down due to gravity. Based on its own weight, if the block applied five
pounds of force down while requiring three pounds of parallel force to slide it across a level surface, then its
coefficient of friction on said surface would be 0.6 (3 ∕ 5 = 0.6).
The idea of an object’s drag factor, however, is somewhat more complicated. A drag factor, like a coefficient of
friction, is actually a non-dimensional number, but is often used in conjunction with the acceleration rate of an object
due to gravity (32.2 ft/sec²). Therefore, many accident reconstructionists add a unit of measurement for gravity
represented by the letter g. So, if a vehicle experiences a 0.6 drag factor while slowing, then it is decelerating at a
rate of about 0.6g or 19.32 ft/sec² (32.2 ft/sec² × 0.6 = 19.32 ft/sec²).
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