Chronology Current Month Current Thread Current Date
[Year List] [Month List (current year)] [Date Index] [Thread Index] [Thread Prev] [Thread Next] [Date Prev] [Date Next]

Re: long and short

This is an excellent point. While at the Atlanta meeting last spring I
attended a medical physics session where one presentation, perhaps the same
guy, but I have forgotten the name now. He stressed that when you are
interested in things like, what happens when the head hits the dashboard, or
a kid tumbles and falls down the stairs compared to being hit by a baseball
bat, etc etc. You can not be as simple minded as looking at just the cm of
the body. You need to model the situation at least as several rigid bodies
that are *articulated*. This can bring about surprising different
conclusions from simpler models. The articulations can create huge
differences in impact speeds for specific body parts due to "slingshot"
affects I presume.

Joel Rauber
Joel_Rauber@sdstate.edu


Hi,
It may be that the original comment was about the
impact of the
head on
the ground. If one assumes that the body falls as a rigid
rod, (and maybe
that
on impact the head acts as loosely connected to the rest of
the body,) I
suspect that the impact velocity might increase quite a bit,
and thus the
energy would also go up quickly.

Thanks
Roger haar

Tim Folkerts wrote:

In the November Scientific American on p. 28, Thomas Samaras is
referenced
as claiming
that a person 20% taller than another, in falling over will hit the
ground
with 210% more
kinetic energy than the shorter person.

Actually, assuming a slight misstatement of the result and
a simple but
questionable assumption, it is fairly straight forward

20% taller => 20% bigger all around [ i.e. the
questionable assumption]
=> (1.2x taller) * (1.2x wider) * (1.2x thicker)
= 1.78 times
heavier

Also, the CM is 1.2 times higher, so PE = (1.78m) g (1.2h)
= 2.07 mgh

So, if KE(final) = PE(initial) we have ~210% times the original KE
{which
is NOT 210% more KE, it's just 110% more}.

Tim Folkerts