Skid-Mark Formula
Definition
v = sqrt(2 mu g d), where v is the pre-skid speed (m/s), mu is the coefficient of kinetic friction between tyre and road surface, g is gravitational acceleration (9.81 m/s2), and d is the measured skid-mark length in metres.
- Formula
- v = sqrt(2 x mu x g x d)
- g value used
- 9.81 m/s2
- Variable mu
- Coefficient of kinetic friction, tyre to road
- Variable d
- Measured skid-mark length in metres
Common questions
What determines the value used for mu?+
It depends on the road surface material and condition, such as dry or wet asphalt versus gravel, and is usually taken from published reference tables or measured directly at the scene with a drag sled.
How is the result converted for a report?+
The formula gives speed in metres per second, which is multiplied by 3.6 to express it in kilometres per hour, a form more familiar to a court or investigator.
Related terms
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