Some Simplified Momentum Assumption Misconceptions

[MSI]

New: Use this link to go to the latest post on this thread

Some Simplified Momentum Misconceptions:

• And please note this is not to dismiss simplified momentum analysis, it is to help all who use it to understand the assumptions, to encourage that they test a range of assumed input numbers to reveal any sensitivities, and then that they try to test and refine any conclusions.

1. The collision between two vehicles is not the same as a collision between two pool balls.
   
   • More like two crushable rectangular boxes on wheels. And vehicles are not 'balls' or 'points',
   • they are finite dimensioned crushable objects which may behave quite differently during interaction than simple `balls' or `point masses'.
2. The assumption of the impulse of momentum acting though the 'centroid of the damage area' is an assumption which was created for mathematical convenience as part of the original development of the CRASH program for NHTSA.
   
   • The centroid of damage for two vehicles may not be coincident at the assumed `point' of exchange of momentum.
   • The actual area of momentum exchange between two vehicles depends on the impact configuration and sometimes the `centroid of the damage area' may not be a good assumption.
3. The actual exchange of momentum between two vehicles takes place along the collision interface.
   • And that damage, observable after the collision, may not have been created in a single `instant'.
   • Think of it as 50 to 150 or MORE "instantaneous exchanges" as the collision occurs and momentum is exchanged...it takes times, movement and distance to occur....NOT a single instant!
4. The exchange does not take place at a single point or in a single direction or in a single instant, that is a simplifying assumption used by momentum solution procedures for convenience.
   
   • The magnitudes of the forces and the moments change with time for the 50 to 100 milliseconds or more of collision interaction.
5. The collision between two vehicles takes time (50 to 150 milliseconds or more), and during that time, the collision partners can change orientation (Depends on the impact configuration, speeds, etc.).
   
   • So in essence during a collision you have 50-150 or more 'instantaneous exchanges' every millisecond!
   • And 'instantaneous momentum exchange simulation' programs lump this into a single point/instant/position for mathematical convenience?
6. There may also be a side-slap secondary contact between vehicles (like during intersection collisions) where after the initial contact of the front corners the vehicles 'slap' sides.
   • This obviously must be considered in any momentum solution procedure and obviously it does NOT occur in an instant.
7. The use of a point ('the centroid'), a single direction (PDOF) and an instantaneous exchange time is for mathematical convenience and simplification.
   
   • Variations of these assumptions should be considered and tested as part of any analysis.
8. The assumption for the direction of the momentum vectors at separation should also consider the rotation directions of the collision partners, the amount of rotation, and the end of rotation position (which may or may not be the position of rest).
   
   • A rotating vehicle does not travel in a straight line, it follows a curved path.

These items and others related to accident reconstruction momentum calculations emphasize the need to test and bracket the sensitivity of your assumptions and to focus on the variable which may have the most profound effect on the results of your momentum analysis.

• When CRASH was created for NHTSA it was as a preprocessor for SMAC.
• NHTSA decided CRASH was adequate FOR STATISTICAL STUDIES where they were looking for trends.
  NOW 40+ years later 'experts' hide these simplifications behind high end graphics and flashy presentation capabilities in INDIVIDUAL CASES??
  Think about that for a moment!...
  

Sept 2020 NOTE: These comments on Momentum analysis apply to momentum in general when applied to motor vehicle crashes and to programs like PC-CRASH, Virtual Crash, Planar Impact Models, and others:

• There are many great programs out there with amazing options/graphics.
• This is just a word of warning to be very careful as momentum and momentum crash simulation programs include the simplifying assumption of an 'instantaneous exchange of momentum' which requires the user select a subjective 'point and angle' for that instantaneous exchange
  • begs question: Why isn't it automatic?
  
  These programs are useful however they can be very sensitive in many impact configurations.
  CHECK FOR SENSITIVITIES OF ANY RESULTS!
  Here are some related articles:
  • Nov 2020 Question on Cone of Departure in Momentum from another forum demonstrating issues of 'instantaneous momentum exchange' particularity when there is a sideslap
    • Nov 2020 What is a Sideslap?
  • Nov 2020 PC-CRASH: How Many % May An Investigator Be Wrong with pc-crash? Question posed on another forum
  • Sep 2020 A Negative Coefficient of Restitution? An Irresponsible vCrash Video! Silly Irresponsible video which caused confusion
  • Aug 2020 More Discussion on PC-CRASH type simplified momentum solutions Review of a pay-per-view pc-crash training video
  • Jun 2020 Does Momentum Analysis Requires COM Common Velocity? Some alleged 'experts' run fast and loose with science
  • May 2020 Planar Impact Model assumptions Planar impact models also have issues