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DeltaV in terms of G's?

Posted: Fri Jun 19, 2009 10:07 am
by MSI
Q: I am used to discussing Delta -V for the forces on a person in a crash. Another expert is using G's. What is the conversion from one to the other?

A: DeltaV, or Impact speed change, is normally associated with a collision. (to avoid those among you who will say `but you encounter a DeltaV when braking!)
  • So it normally occurs over a 50 to 200 millisecond period of time.
    DeltaV is an impulse, which means a Force over time.
    From Sir Isaac, F=Ma.
    So the acceleration is the Force/Mass.
    So DeltaV*Mass = Force * DeltaT
    An impact speed change generally does not occur at a constant force. It's waveform is generally considered similar to a quarter sine wave (there have been a few recent papers looking at different characterizations)
    So what does that mean?
    Well that the force, or acceleration, is not a constant.
    So to characterize an impact speed change you need to know the duration of the impulse and characteristics of the force (or acceleration) during that impulse.
When someone quotes acceleration, or G's, you need to find out:
  • Do they mean average acceleration (meaning assuming a square wave?)
    Do they mean peak acceleration?
    And if peak then you must know what waveform are they assuming for the acceleration (so you can find the area under the curve or waveform)?
For a interesting and timeless discussion of Energy and Force considerations, see the APPENDIX- DAMAGE INTERPRETATIONS of the paper by Raymond R. McHenry "Computer Aids for Accident Reconstruction" SAE Paper 760776.

It is not only about damage interpretations, it is about work, energy, acceleration, etc.