Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!usc!cs.utexas.edu!uunet!lhdsy1!yzarn
From: yzarn@lhdsy1.chevron.com (Philip Yzarn de Louraille)
Newsgroups: rec.skydiving
Subject: Re: Graph of osc'n due to high windforce
Message-ID: <1025@lhdsy1.chevron.com>
Date: 30 Jun 91 03:26:36 GMT
References: <1991Jun24.154627.285@Stardent.COM> <997@lhdsy1.chevron.com> <1991Jun26.182628.1006@Stardent.COM>
Organization: Chevron Oil Field Research, La Habra, CA.
Lines: 51

In article <1991Jun26.182628.1006@Stardent.COM> joep@Stardent.COM (Joe Peterson) writes:
>> Say the parachute takes 2 seconds to open 43/2 = 21.5 m/s/s
>> Since g = 9.81 m/s/s, the deceleration is 21.5/9.81 [m/s/s*g/(m/s/s)]
>> = 2.2 g deceleration. Yes, it is above one, but only because the amount
>> of time it took to slow down was short. And this is the net difference.
>> Now, I agree that the net upward force on a skydiver in freefall in an
>> atmosphere (Earth!) will be greater than 1 g, in the case discussed
>> previously (a few exchanges ago), it would be around 1.025 g?
>> *But* 1 g would be cancelled because of the net downward gravitational
>> attraction. So the skydiver would *feel* 0.025 g which is unfeelable by human
>> standards!
>
>Hmmm, you say it is above 1.0 only because the time is short.  That is
>not true.  It would be greater than 1.0 even if the time were very very
>long (anything short of infinite!) but would be very close to 1.0 (in fact,
>I agree that it would be too small to notice).  When you say the 1.0 g
>would be cancelled out, I think you are normalizing the 1.0 g we feel
>on the ground.  The force is still there, but it feels normal!

If a person were to experience a total acceleration greater than one g
directed upwards, after a certain time (which would be short and depend
on the original downward velocity, say 5-10 sec) that person would
accelerate towards the sky because the Earth would compensate for only 
*one g*. But this does not happen because the total acceleration is
less than one g and directed upwards because the skydiver slows down
(since the atmosphere is getting thicker.)
>
>> A person at rest on the ground does not pull one g! Yet this is what you
>> argue a skydiver feels in freefall. I disagree.
>
>Why not?  Is the person not feeling normal gravitational force?  Granted,
>this force is directed vertically, and when people talk about "pulling
>g's," they usually are referring to force along an arbitrary axis caused
>by some vehicle.  Have you ever done "zero g's" in the jump plane?  This
>is because the plane starts to follow the same parabolic path you do
>as you truely freefall (since we can really ignore air friction now!).
>
I think I finally understand why we disagree. I think you are confusing
acceleration and force. They are not the same thing. I agree (how could
I not) that Mother Earth directs on me a force, but I am not
experiencing an acceleration of any kind (including one g) when I am
standing still.
Once again, what is important when you analyze a system is the resultant
or summation of all forces, not just one. The summation of the
acceleration is not the concept to use to analyze such systems. This is
Newton's first law.
-- 
  Philip Yzarn de Louraille                 Internet: yzarn@chevron.com
  Research Support Division                 Unix & Open Systems
  Chevron Information & Technology Co.      Tel: (213) 694-9232
  P.O. Box 446, La Habra, CA 90633-0446     Fax: (213) 694-7709