Newsgroups: rec.skydiving
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From: jnrees@athena.mit.edu (Jim Rees)
Subject: Re: Graph of osc'n due to high windforce
Message-ID: <1991Jun26.162046.8601@athena.mit.edu>
Sender: news@athena.mit.edu (News system)
Organization: Massachusetts Institute of Technology
References: <983@lhdsy1.chevron.com> <1991Jun24.153922.12763@rodan.acs.syr.edu> <1000@lhdsy1.chevron.com>
Date: Wed, 26 Jun 91 16:20:46 GMT
Lines: 67

In article <1000@lhdsy1.chevron.com> you write:
>I strongly disagree and let see if I can make you change your mind.
>In the absence of atmosphere, the potential energy (of a falling object)
>would be changed into kinetic energy. Do you agree?
>If so, then, since the atmosphere does not "know" that a body is there,
>how could the mere presence of an atmosphere change the laws of physics?
>How could the potential energy of the falling object suddenly stop being
>transferred into kinetic energy and instead change to be transferred
>into heat? (well, according to your model, the potential energy would
>gradually transfer itself from kinetic to heat, as "terminal" velocity
>would be reached.) I just don't see a law of physics that would allow
>that.

No laws of physics are being broken here. Without atmosphere, there is
only one force on the skydiver,  so he accelerates without bound until
he hits the ground.   With an atmosphere, the skydiver's acceleration
starts at 9.8 m/s^2 and gradually reduces to zero when he hits
terminal velocity.  At this point, the skydiver's kinetic energy is no
longer increasing, but his potential energy is decreasing as he falls.
Where is this energy going?  Since the skydiver isn't getting any of
it, it must be going into the air in the form of heat, sound,
turbulence, etc.   Now in between exiting the aircraft and hitting
terminal velocity, the skydiver's kinetic energy is increasing, but
not as fast as his potential energy is decreasing.  The difference is
transferred to the air.

>Also, if the potential energy was not transformed into kinetic energy
>(or stopped being transformed into kinetic energy and instead transfered
>itself into heat) then the skydiver would slow down due to the friction
>with the atmosphere. 

No, because nothing is taking away the kinetic energy that the
skydiver has once he has reached terminal.  The friction with the
atmosphere only prevents the skydiver from gaining any *additional*
kinetic energy.

>>assumption.  Maybe I'm too used to feeling that 1 g upward acceleration under
>>my feet every day. :)
>
>Really? Are you feeling 1 g all the times? (or are you just kidding?) ;-)
>-- 

Yes, one *feels* the push of the ground.  Have you ever made a balloon
jump?  You know that weird feeling when you first step off when there
is negligible air friction.  That is the *feel* of no forces, even
though there is a net force of 1g.  The reason why gravity cannot be
felt is because it acts uniformly on all the particles in our body.
Our entire body accellerates uniformly in a gravitational field.  On
the ground, however, the normal force the ground provides to prevent
us from accellerating towards the center of the earth does not act
uniformly on our bodies.  There is much more pressure in your feet,
for example, than your head.  

When you see pictures of astronauts in the space shuttle, they float
around, apparently experiencing a zero-g environment.  But in fact,
there is still a force of 1g on them (ok, since they're further from
the center of the earth by a couple percent, the force is a little
less than 1g if 1g is defined as the gravitational force at ground
level).  They are, at all times, accelerating uniformly towards the
center of the earth, but never get any closer since they're in orbit.
Other than the force of gravity, there are no other significant forces
on the astronauts, and they feel like they're in a zero-g environment
(which, by the way, would feel the same way).


	Jim Rees D-13359