Conservation of Linear Momentum

The Conservation of Linear Momentum method is a well established scientific too utilized by the reconstructionist to determine the impact speeds of vehicles involved in collisions. It is one of the many useful techniques that an accident reconstructionist has at their disposal. Others include Conservation of Energy, Critical Speed Analysis, and the Crush Factor Method. Conservation of Linear Momentum utilizes several inputs that require careful evidence collection and analysis. Additionally, some of these inputs are more sensitive than others depending on the particular situation. Next we will list the inputs and discuss some of the methods used for determining the proper value to use.

Needed Inputs:
  • Angle at Start of Post Impact Rotation
  • Angle at End of Post Impact Rotation
  • Lock-up value per Wheel
  • Weight distribution per Wheel
  • Post Impact Drag Factor
  • Post Impact Travel Distance
  • Grade in Area of Post Impact Travel
  • Approach Angle
  • Departure Angle
  • Total Weight
Accurate documentation of the vehicles, roadway, and crash scene markings is necessary. This can be done with total station technology or some other accurate method of measurement. A drag sled can be used to determine drag factors on various surfaces. Other methods such as skid tests or drag factor tables in certain instances are necessary. This documentation which is used to make an accurate scale diagram is then utilized to obtain values for the above variables. Data from the vehicle and crash scene markings will be used to determine the inputs for the lock up factor which in turn will be used for the departure speeds. Other values like the weights of the vehicles are normally obtained from professional databases like expert auto stats and then combined with the estimated weights of the occupants and cargo.

Conservation of Linear Momentum analysis, which is a scientifically sound method used by reconstructionists for calculating impact speeds and other information about crash vehicles, yields good information when proper data is inputted. The method requires several inputs some of which may be sensitive. Consequently, it is imperative that a detailed analysis and an experienced professional evaluation of the data be performed to obtain accurate results.

Todd Hutchison

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