Does Momentum Analysis Requires COM Common Velocity?

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MSI
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Does Momentum Analysis Requires COM Common Velocity?

Post by MSI »

Q: I have an intersection collision, vehicle turning left struck by a through vehicle.
  • The opposing expert claims that momentum cannot be used to calculate impact speeds. The claim is because the vehicles centers of mass (COM) did not reach a common velocity the collision does not meet the requirements for conservation of momentum. The two vehicles are similar in size, shape, and weight. The collision occurred at the back right door of the struck left turning vehicle with that vehicle traveling approximately 40 feet post impact while rotating approximately 250 degrees clockwise before coming to uncontrolled rest. The bullet vehicle continued over 160 feet before coming to uncontrolled rest. Am I missing something?
    I don't remember any discussion about the centers of mass needing to reach a common velocity for conservation of momentum being applied, only that a point within the contact area needed to reach a common velocity. Understandably, this does not apply to a sideswipe type collision but this is far from a sideswipe collision.
Is anyone aware of any literature that discusses this?

A: I concur that unless the collision was a side-slap or has some other limiting factor a Momentum analysis can be used.
Realize that Linear Momentum does has some limitations,
For example,
  • the use of the conservation of Linear Momentum requires first that the directions of motion of the two vehicles at impact (which is generally assumed to also be separation) are accurately defined
    • in a prior case we had estimates of directions used by an expert to include the unrealistic assumption that the direction of motion of one of the vehicles did NOT change by the collision.
  • I would also suggest performing a sensitivity study to small variations in the assumed directions of motion (Think Monte Carlo! See Wades's write-up: An article on using Monte Carlo Analysis) to see if any variables are extremely sensitive for the impact configuration.
  • Next, how did they determine the direction and velocities of the vehicles at separation from impact and are the speeds and directions adequate to travel to the proper positions of rest?
  • And note Linear momentum solutions do not include any test that the impact configuration produces the proper and adequate angular velocities at separation to produce the proper travel and rotation from separation to positions of rest.
I always suggest some additional analyses based on either
BTW what analysis did this 'expert' suggest to use instead of Momentum? Sir Isaac asks and is wondering?
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MSI
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Posts: 1694
Joined: Thu Jun 18, 2009 12:37 pm

Re: Does Momentum Analysis Requires COM Common Velocity?

Post by MSI »

A response from poster:
Q: I did calculate impact speeds using the Monte Carlo method and then simulated the collision with PC-Crash. The results of both were very well matched and the PC-Crash simulation results were very similar to the actual crash event, the post impact trajectories, travel, and rest positions.
You can let Sir Isaac know that the other "expert" did a very elementary analysis of the crash by assigning post impact drag factors, approximating (guessing) at a speed loss range relative to the bullet vehicle from the damage the vehicle sustained, and combining the speeds.

A: Always love it when an expert uses their 'scientifically calibrated and no doubt self proven' WAG* theory....
Kinda sorta makes our job easier when the 'expert proof' is wetting their thumb and raising it above their head to see...
which way does thoust wind blow?...

*Wild A$$ Guess...the chosen technique for many of the 'You Pay, We Say' type experts...closely related to the oldest profession!! ;-)
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