Superfreaks/ACRA: Superfreaks Season 3 #1

Eagle eagle at
Sat Oct 6 11:20:50 PDT 2007

Martin <phippsmartin at> writes:
> On Oct 6, 6:54 am, Eagle <eagle at> wrote:

>> Assuming the bullet does not add appreciably to the mass of the head,
>> the worst-case for head movement would be a complete momentum transfer
>> (no exit wound), which would result in a head speed immediately after
>> inelastic collision of 4.2 m/s, or 9 mph.  That means that in this
>> worst case (which isn't correct, since we know there was an exit wound
>> and therefore the bullet didn't transfer all of its momentum to the
>> head), you would expect as much head movement as you'd get from a
>> sudden decrease in the speed of the car of about 10 mph.  That's some,
>> but not much, and the neck is likely to absorb a lot of that momentum.

> Actually, no.  When a car decreases suddenly by about 10 mph, the
> entire body moves forward a bit so you wouldn't notice any movement of
> the head compared to the rest of the body.

Your analysis is somewhat wrong:  the body does not move as a rigid object
and you can indeed notice head movement independent of the body in an
abrupt acceleration because the head has its own momentum and the neck
flexes.  But the point that the car is a bit too easy of an analogy is
well-taken.  Some of the same points also apply to the bullet impact, but
analysis becomes tricky because you have to account for the degree to
which parts of the body move independently of each other.

The underlying point is more that the actual momentum transfer is much
smaller than you'd think (particularly given that there was an exit wound
and hence only partial momentum transfer).  In practice, it would surprise
me if the actual induced velocity would have been over 3mph, even had the
bullet hit in such a way as to let the head move fully independently of
the rest of the body, which it didn't.

Eagle (eagle at                            Windrider of Crossroads
<>          rec.arts.comics.creative moderator

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