Momentum "Cone of Departure" Question

General Questions related to the Momentum Based Analysis programs
#pc-crash #virtualcrash #crash
MSI
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Momentum "Cone of Departure" Question

Post by MSI »

Dec 16, 2020 this Question, from another forum, was originally posted to Some Momentum Misconceptions, however once we saw a full scale test on the topic we spun if off to a separate topic.
On our response to this thread (see below) we posted a video so why not bring it to the top,
read through this you'll see why we posted this video:
Question on another forum: (INCR, join that forum!)
  • In a book under Myths and Misconceptions there is a discussion of the Cone of Departure in Momentum analyses. The book describes and gives examples that there is no requirement that the departure angles of two colliding vehicles be within the approach heading vectors.
    My questions is, has anyone seen this happen from an actual crash, staged or otherwise?
    If so, what were the circumstances?
    Any reference location on this topic would be appreciated.
Answer posted to the forum:
  • Newton requires that the SUM of the momentum vectors be the same incoming and outgoing. Newton does NOT care if they are in the "cone of departure" on leaving or not, though...just that the sum is pointed in the same direction. If there is a big enough momentum disparity and the correct angle relationships, the sum can still be in the cone, while one of them is, by itself, NOT directed inside the cone. It is possible. We did it at the Joint Conference a few years ago. Bruno Schmidt and I published an article in Collision - Collision Magazine 5(2), Fall 2010 with the title: Cone of Departure: A Good Idea, but NOT a Law about it.
Our RESPONSE ABOUT "CONE OF DEPARTURE"
  • Great article (quick read looks good) and when i get a chance for a more detailed review i might also create a simulation of the collision example to demonstrate the issue i always harp about (sorry folks!) with respect to simplified momentum collision reconstruction.

    That is:
    Collisions do NOT happen in an instant...actually 50 to 150 or more milliseconds or "instants"
    • and so really a collision is a
      • 50 to 150 millisecond collisions.(and a millisecond is closer to an 'instant' than a full collision!)
      • I hadn't thought of characterizing it as 50-150 instantaneous momentum exchanges, but that is what it is!
      Because simplified momentum interaction tries to approximate a finite collision interaction (50 to 150 milliseconds of collision interactions)
      the "cone of departure" at the initial impact and then the "cone of departure" at the point of separation will be different as the vehicles move, rotate, crush, etc (And let us not forget sideslaps!!)

      Main points:
      • The collision of two vehicles is not instantaneous and they are not point masses.
      • All simplified momentum analyses, which assume instantaneous exchange, whether hand calculations or sophisticated computer programs, are good in many instances
        However
      • They are trying to approximate 50 to 100 'instantaneous' collisions in a single instant.
      Understanding that will help you understand why they don't always seem to 'be in the cone' and therefore make you scratch your head.
    Please also see:

This topic has 2 more posts with additional information

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