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Some Momentum Misconceptions

Posted: Fri Jun 19, 2009 11:11 am
by MSI
Some comments on Momentum analysis:
  • (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 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 f 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'. The magnitudes of the forces and the moments are changing with time.

    (4) The exchange does not take place at a single point or in a single direction or in a single instant, that is an assumption for convenience.

    (5) 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).

    (6) There may also be a side-slap which obviously must be considered in the exchange of momentum and obviously does not occur in an instant.

    (7) The use of a point ('centroid') and a single direction (PDOF) and an instantaneous exchange time is for mathematical convenience and variations of those assumptions should be considered and tested as part of any analysis.

    (8) The assumption for the direction of the momentum vectors at separation should consider the rotation direction, 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.

Re: Some Momentum Misconceptions

Posted: Wed Aug 12, 2009 10:47 am
by brian
This is some additional information on item:
  • (6) above "There may also be a side-slap which obviously must be considered in the exchange of momentum and obviously does not occur in an instant".
There's something about sideslaps!
Rusty Haight sent us some information on the 2007 NYSTARS sideslap crash, (thank you Rusty!) and we did a momentum analysis and SMAC simulation of the collision as a part of our 2008 McHenry Software seminar.
  • Above is a snip of a simulation of the collision with the velocity vectors displayed
    We have also posted up a graphic and tabular summary of the results which includes the positions, velocities, momentum and kinetic energy calculations
    Notice how the velocity vectors direction and magnitude changes during the sideslap. Notice also that the time between initial contact and separation from the sideslap is >> instantaneous!
So for a simplified momentum analysis, which angle do you use and how can you assume 'instantaneous' when the tires are skidding and changing the speed between initial contact and separation from the sideslap?
Obviously knowing the answer, you can judiciously pick an angle that 'works'.
However if you 'do not know' the answer you want or 'need' (seems some folks do reconstruction on an agenda rather than as a scientific endeavor (i'll have more on that later!)), then you will need to use a tool such as SMAC to test and refine your simplified momentum analysis.
SMAC is the only tool available which can scientifically and objectively handle collisions which include a sideslap.

Re: Some Momentum Misconceptions

Posted: Thu Oct 08, 2009 12:49 pm
by brian
In re-reading my response, one might interpret my message to mean you MUST run SMAC for every accident reconstruction.
That is not what i meant.
The main point of emphasis with respect to Momentum Solutions is to perform a sensitivity study.
Far too often we encounter ‘experts’ who present a single momentum solution to a complicated collision. When we vary some of the assumptions, such as the assumption of exit angle or exit speed, we get a wide variation in results.
Therefore the ‘single’ solution is not a valid solution.
So as a minimum, someone using a Momentum Solution should do a sensitivity study to determine the range for their results.
And then present the results as a range of results.
There have also papers on using a Monte-Carlo solution procedure to determine sensitivity areas. I will post more on that in another post.