### Some Momentum Misconceptions

Posted:

**Fri Jun 19, 2009 11:11 am**

**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 bein many impact configurations. CHECK FOR SENSITIVITIES IN RESULTS!*very sensitive*

Here are some related articles:*PC-CRASH: How Many % May An Investigator Be Wrong with pc-crash? response to a question posed on another forum**More Discussion on PC-CRASH type simplified momentum solutions review of a $100 pc-crash training video**Does Momentum Analysis Requires COM Common Velocity? Some alleged 'experts' run fast and lose with science**Planar Impact Model assumptions planar impact models have issues with simplifying assumptions and subjective inputs*

**Some Momentum Misconceptions:**

- 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'.
- 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 development of CRASH. The centroid of damage for two vehicles may not coincide 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.
- 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'. The magnitudes of the forces and the moments are changing with time.
- The exchange does not take place at a single point or in a single direction or in a single instant, that is an assumption used by momentum solution procedures for convenience.
- The collision between two vehicles takes time (50 to 150 milliseconds or so), and during that time, the collision partners can change orientation (and again, how much depends on the impact configuration, speeds, etc.).
- 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.
- The use of a point ('centroid') and a single direction (PDOF) and an instantaneous exchange time is for mathematical convenience. Any variations of those assumptions should be considered and tested as part of any analysis.
- 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.