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From: greeny@wotan.top.cis.syr.edu (Jonathan Greenfield)
Newsgroups: rec.skydiving
Subject: Re: Graph of osc'n due to high windforce
Message-ID: <1991Jun24.153922.12763@rodan.acs.syr.edu>
Date: 24 Jun 91 19:39:22 GMT
References: <1991Jun14.115812.28436@rodan.acs.syr.edu> <970@lhdsy1.chevron.com> <1991Jun18.150541.5220@rodan.acs.syr.edu> <983@lhdsy1.chevron.com>
Reply-To: greeny@top.cis.syr.edu (Jonathan Greenfield)
Organization: CIS Dept., Syracuse University
Lines: 58

In article <983@lhdsy1.chevron.com> yzarn@lhdsy1.chevron.com (Philip Yzarn de Louraille) writes:

>>Actually, the "horizontal" energy is kinetic.  In any case, even after the
>>diver reaches "terminal" velocity, he is inputting energy--he keeps giving
>>up potential energy by falling (which is not replaced with kinetic energy,
>>since his velocity is "terminal").
>
>Yes, you are right, the horizontal energy is kinetic.
>No, you are wrong about the potential energy *not* being transformed
>into kinetic energy. A simple answer to that is "The Law of
>Conservation of Energy". A more physical answer is: the reason why the
>velocity is terminal is due to friction with the atmosphere. The
>potential energy is being transformed in kinetic energy which, in turns,
>is transfered as heat due to the skydiver "frictionning" with the
>atmosphere! What happens is that the skydiver gives up the *kinetic*
>energy, not the potential energy, because it is first transfered to
>kinetic, then heat.

I disagree.  Of course energy is conserved--but in the system (diver +
atmosphere), not just in the diver.  As I stated in a previous post, the 
potential energy is lost via drag to heat, sound, etc.  However, it cannot
be that the energy is first transformed into the skydiver's own kinetic energy.
As long as the diver's velocity is fixed, he cannot be gaining any kinetic
energy (KE = 0.5*mass*velocity^2).  Molecules in the atmosphere, of course, 
can gain kinetic energy.

>>>The net freefall deceleration of 750 mph to 90mph (at 2000') is very
>>>slow and I doubt that it can be felt. After all, in a jump from 12000',
>>>skydivers do not feel their terminal velocities decreasing (until they
>>>open their chutes!), yet this is the region where the increase of
>>>density per altitude change is the greatest (logarithmic profile of
>>>atmospheric density.)
>>
>>You are undoubtedly correct.  I was simply trying to establish that an
>>upward acceleration of >1 g is reasonable.  I don't mean to make any claims
>>about exactly how large that acceleration will be.  (I don't know enough about
>>the atmosphere.)
>
>No, the upward acceleration you come up with is wrong, it is not greater
>than 1 g, that would be quite noticeable!

This is just a misunderstanding...sorry that I was not clearer.  I did not mean
that there was a *net* upward acceleration >1 g--simply an upward acceleration
(working against gravitational acceleratIon).  My qualitative argument only 
supported that it is possible to have a *net* upward acceleration.  Since I 
have no quantitative data regarding the atmosphere, I couldn't say anything 
more than that.

Apparently, I misunderstood your original post.  I assumed that the figures
being quoted had not yet been adjusted to reflect the downward acceleration
of gravity (and, therefore, a net acceleration).  Looks like that was a poor
assumption.  Maybe I'm too used to feeling that 1 g upward acceleration under
my feet every day. :)


greeny                                           greeny@top.cis.syr.edu

"What's the difference between an orange?"