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From: joep@Stardent.COM (Joe Peterson)
Newsgroups: rec.skydiving
Subject: Re: Graph of osc'n due to high windforce
Message-ID: <1991Jun26.182628.1006@Stardent.COM>
Date: 26 Jun 91 18:26:28 GMT
References: <1991Jun21.145113.6028@Stardent.COM> <994@lhdsy1.chevron.com> <1991Jun24.154627.285@Stardent.COM> <997@lhdsy1.chevron.com>
Organization: Stardent Computer, Concord MA
Lines: 57
In-reply-to: yzarn@lhdsy1.chevron.com's message of 25 Jun 91 18:00:59 GMT

> 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!

> When I am standing up, Earth is pulling down 1 g, the surface I am
> standing on compensates (so I don't sink into the floor) and I will
> argue that I don't feel the floor pushing me up (to avoid sinking into
> it!) I can also say that I do not feel my weight either because my
> muscles are used to and can compensate (for now!) for the gravitational
> pull.

I think we are confusing "experiences" here.  Since it is natural to
continuously feel normal gravity, people normalize this experience and
think of forces in relation to it.  But I still maintain that you feel
the force of your weight (but as you say, you ignore it).
When you are standing up, don't you feel the floor "pushing up?"  If you
have ever used a leg press weight machine, you know the sensation of the
machine pushing your feet toward you.  In this case, you would be working
against the force of the weight you selected instead of your own weight.
If you were weightless for a while (zero g's), a return to normal gravity
would certainly make you aware of your weight!

> 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 look forward to your next entry, this is stimulating.

Cool!

> E.F.S.

Absolutely!

					Joe Peterson
					C-20351
					joep@stardent.com