http://www.nasa.gov/centers/glenn/research/microgex.html
http://en.wikipedia.org/wiki/Weightlessness
I realized that I misunderstood the notion of microgravity in space and
the fact that it is created in LEO as opposed to naturally existing.
In the first link on microgravity it became clear after reading this
paragragh:
Many people mistakenly think that there is no gravity above the Earth's
atmosphere, i.e., in "space," and this is why there appears to be no
gravity aboard orbiting spacecraft. Typical orbital altitudes for human
spaceflight vary between 120 - 360 miles (192 to 576 km) above the
surface of the Earth. The gravitational field is still quite strong in
these regions, since this is only about 1.8% the distance to the Moon.
The Earth's gravitational field at about 250 miles (400 km) above the
surface maintains 88.8% of its strength at the surface. Therefore,
orbiting spacecraft, like the Space Shuttle or Space Station, are kept
in orbit around the Earth by gravity.
The part about being motionless at 250 mi. above the earth's surface
and still having 88.8% gravity did it.
Also, in the second link, this part:
As a thought experiment, imagine a spacecraft that had the ability to
rise up to orbital altitude by going straight up like a helicopter and
hovered over one spot on the Earth. The astronauts inside would not
experience weightlessness. Their ride inside this hovering spacecraft
would be similar to riding an elevator up an incredibly tall building
and stopping at the top floor. While hovering above Earth's atmosphere,
their weight would be very close to what they weigh on the surface of
the Earth, even as a space shuttle goes zinging by them. So astronauts
in a hovering spacecraft are being pulled by strong gravity just as
space shuttle astronauts are pulled by strong gravity. The difference
between them is that the orbiting shuttle is freely being pulled toward
the center of the Earth. The lack of relative acceleration between the
orbiting shuttle and its astronauts inside (who are also being freely
pulled toward the center of the Earth) result in them being weightless.
But the hovering spacecraft (as with an elevator at the top of an
incredibly tall building) is not freely falling. The pull of gravity it
is experiencing is being opposed by the hovering force. This force gets
transfered to the astronauts within (along with everything else within
the spacecraft) resulting in weight. This example illustrates the fact
that there is plenty of gravity out in space. If you were to take any
object that is orbiting the Earth and stop it dead in its track and
then release it, the Earth's gravity would pull it straight down back
toward the Earth's surface.
Yep, very clear there.
Eric
This is why you can experience microgravity in an aircraft flying parabolas.
Wasn't it Newton who had the thought experiment about firing a canon on top
of a mountain that extended above the atmosphere? For small powder loads,
you got the expected parabolic shape of the shell falling to the earth. But
as you kept increasing the powder load, eventually you get to the point
where the shell falls all the way around the earth. That's an orbit.
This is also why microgravity is sometimes called free fall.
We mere mortals can experience free fall for a few seconds by riding drop
tower like rides at theme parks. There's one a few miles from my home at
King's Island in Mason Ohio. It's lots of fun, really it is! ;-)
Jeff
--
"They that can give up essential liberty to obtain a
little temporary safety deserve neither liberty nor
safety"
- B. Franklin, Bartlett's Familiar Quotations (1919)
>
>"Eric Chomko" <pne.c...@verizon.net> wrote in message
>news:1162241128.9...@b28g2000cwb.googlegroups.com...
>> Why reading the following two pages:
>>
>> http://www.nasa.gov/centers/glenn/research/microgex.html
>> http://en.wikipedia.org/wiki/Weightlessness
>>
>> I realized that I misunderstood the notion of microgravity in space and
>> the fact that it is created in LEO as opposed to naturally existing.
>
>This is why you can experience microgravity in an aircraft flying parabolas.
>Wasn't it Newton who had the thought experiment about firing a canon on top
>of a mountain that extended above the atmosphere? For small powder loads,
>you got the expected parabolic shape of the shell falling to the earth. But
>as you kept increasing the powder load, eventually you get to the point
>where the shell falls all the way around the earth. That's an orbit.
Also note that it's never a true parabola (as it would be in a uniform
gravitational field, which doesn't actually exist anywhere in the
universe), but for small distances it approximates one. It's always a
partial ellipse.
Quite true. I forgot about that little detail since for small distances it
is rather convenient to approximate the path as parabolic (high school
physics). Still, the thought experiment is an easy way to introduce the
theory of orbits to people without resorting to math which may scare them
off.
Here's a link to a picture for those that would like to see it:
http://www.waowen.screaming.net/revision/force&motion/ncanon.htm
And an interactive web page with animation for the video game generation:
http://www.waowen.screaming.net/revision/force&motion/ncananim.htm
This thought experiment can also be used to visualize why something like a
cannon can't be used to place something in orbit, at least not without a
rocket engine to circularize the orbit so the projectile doesn't come back
and hit the earth after one "orbit".
You just can't do it my measuring at a single point --
but you can, if you can measure at two separate points in the
room.
Yes.
Yet note that for any given sensitivity of measurement and separation of
measuring points, there is a distance-X-to-mass-center which can't be
told from a distance-Y-to-spin-center case. And (per other threads)
this can't-be-told-from quickly falls below physiological sensitivity
(well... fairly quickly). Specifically, your heart and circulatory
system really can't tell the difference between pushing blood up an
acceleration potential and pushing it up a gravitational potential.
Because, in terms of energy, fluid pressure, and physiological effects,
there *is* no difference.
Wayne Throop thr...@sheol.org http://sheol.org/throopw
>: "Jim Oberg" <james...@houston.rr.com>
Right. We're not talking about practical differences. We're talking
about "Einsteinian equivalence" differences (note: the elevator
thought experiment is always careful to state that it's impossible to
tell the difference between an acceleration and a "uniform
gravitational field"--it if gets it right, that is...).
Jeff Findley wrote:
>Wasn't it Newton who had the thought experiment about firing a canon on top
>of a mountain that extended above the atmosphere? For small powder loads,
>you got the expected parabolic shape of the shell falling to the earth. But
>as you kept increasing the powder load, eventually you get to the point
>where the shell falls all the way around the earth. That's an orbit.
>
>
This is what thwarted Herr Scultze's attempt to destroy France-Ville
with his giant cannon at Stahlstadt* in Verne's "The Begum's Fortune".
* Love that name...this was Verne taking a crack at the Krupps, and
their Essen factories. The "City Of Steel" is about as pleasant a place
as one would expect it to be, as was Essen.
Pat
Real gravitational fields are never actually point source equivalents,
so it's never actually a partial ellipse.
The equivalence principle is only really true as the limit for a point.
However, there's various mass layouts that give a pretty constant field
strength over a wide volume to quite high accuracy.
IRC the center of a hemispheric shell is one, and the center of a large
disk is another.
Bob forward pointed out that you can in principle make a zero-gravity
room on the Earth if you could mount a disk of neutronium above it. (The
minor implementation details for doing that are left to the reader ;-) )
Hold it up with scrith? ;-)
> Right. We're not talking about practical differences. We're talking
> about "Einsteinian equivalence" differences (note: the elevator
> thought experiment is always careful to state that it's impossible to
> tell the difference between an acceleration and a "uniform
> gravitational field"--it if gets it right, that is...).
Here you guys might want to check out the sites from the Gravity Probe
B launched april 20 2004 at Nasa, and the site at Stanford where the
scientists will be publishing their results soon.
Tom
http://www.nasa.gov/centers/marshall/news/news/releases/2005/05-160.html
"Launched on April 20, 2004, from Vandenberg Air Force Base, Calif.,
Gravity Probe B has been using four spherical gyroscopes to precisely
measure two extraordinary effects predicted by Einstein's theory. One
is the geodetic effect, the amount by which the Earth warps the local
space time in which it resides. The other, called frame-dragging, is
the amount by which the rotating Earth drags local space time around
with it."
http://einstein.stanford.edu/
"We are now proceeding with Phase III, the final phase-of the data
analysis, which will last until January-February, 2007. Whereas in
Phases I and II the focus was on individual gyro performance, during
Phase III, the data from all four gyros is being integrated over the
entire experiment. The results of this phase will be both individual
and correlated changes in gyro spin axis orientation covering the
entire 50-week experimental period for all four gyros."
Open sharing of information is crucial to improving everybody's
understanding of the universe around us.
tom
` "Ian Woollard" <ian.wo...@gmail.com> wrote in message
` news:4546ffb0$0$8719$ed26...@ptn-nntp-reader02.plus.net...
` > Bob forward pointed out that you can in principle make a
` > zero-gravity room on the Earth if you could mount a disk of
` > neutronium above it. (The minor implementation details for
` > doing that are left to the reader ;-) )
` Hold it up with scrith? ;-)
I think an engineering study would be likely to conclude that a
cheaper alternative would be to dig down and construct the
room at the centre of the earth... (^:
--
==========================================================================
vincent@triumf[munge].ca Pete Vincent
Disclaimer: all I know I learned from reading Usenet.
> In sci.space.policy, on Tue, 31 Oct 2006 07:34:15 -0500,
> Jeff Findley <jeff.f...@ugs.nojunk.com> sez:
>
> ` "Ian Woollard" <ian.wo...@gmail.com> wrote in message
> ` news:4546ffb0$0$8719$ed26...@ptn-nntp-reader02.plus.net...
>
> ` > Bob forward pointed out that you can in principle make a
> ` > zero-gravity room on the Earth if you could mount a disk of
> ` > neutronium above it. (The minor implementation details for
> ` > doing that are left to the reader ;-) )
>
> ` Hold it up with scrith? ;-)
>
> I think an engineering study would be likely to conclude that a
> cheaper alternative would be to dig down and construct the
> room at the centre of the earth... (^:
Dig down?
A signifigant mass of neutronium (disregarding how it would be kept
at that density, once removed from a neutron star [and however that
removal process would be accomplished]) would likely find its way to
Earth's core in short order...just, um, follow it.
--
Frank
You know what to remove to reply...
Check out my web page: http://www.geocities.com/stardolphin1/link2.htm
"To confine our attention to terrestrial matters would be to limit the
human spirit."
- Stephen Hawking
> Bob forward pointed out that you can in principle make a zero-gravity room
> on the Earth if you could mount a disk of neutronium above it. (The minor
> implementation details for doing that are left to the reader ;-) )
Actually, Heinlein mooted this concept, and a few problems with
the idea, in the short story "—We Also Walk Dogs" ...
... in 1941 :)
--
Chuck Stewart
"Anime-style catgirls: Threat? Menace? Or just studying algebra?"
In the Gravity Probe B Stanford website, on the page for technical
papers and under the topic "The Origins of Drag-Free Satellites & the
GP-B Experiment" , there is a great list of sources in support of the
experiment.
Proposal for a Satellite Test of the Coriolis Prediction of General
Relativity
G. E. Pugh. Reprinted in Nonlinear Gravitodynamics, The Lense Thirring
Effect, a documentary introduction to current research. Editors: Remo
J. Ruffini, Costantino Sigismondi, 2002, pp. 414-425. Orininally
Published as U.S. Department of Defense Weapons Systems Evaluation
Group Research Memorandum No. 11, 1959.
Possible New Experimental Test of General Relativity Theory
L. I. Schiff. Physical Review Letters, Vol 4, Number 5, March 1960, pp.
215-217.
Motion of a Gyroscope According to Einstein's Theory of Gravitation
L. I. Schiff. Proceedings of the National Academy of Sciences, Vol. 46,
June 1960, pp. 871-882.
Proceedings of the July 1961 Conference on Experimental Tests of
Theories of Relativity
Held at Stanford University, July 20-21, 1961 and sponsored by the
National Aeronautics and Space Administration, Office of Space
Sciences, headed by Dr. Nancy G. Roman, Chief of Astronomy, Solar
Physics, Geophsics, and Relativity programs. The conference was chaired
by H. P. Robertson, Professor of Physics at the California Institute of
Technology, and participants included over 30 well-known authorities in
physics and aerospace engineering.
Requirements and Design for a Special Gyro for Measuring General
Relativity Effects from an Astronomical Satellite
Robert H. Cannon, Jr., Chairman of the Department of Aeronautics &
Astronautics, Stanford University, 1962. Published in the Proceedings
of the International Union of Theoretical and Applied Mechanics,
Kreiselproleme Gyrodynamics, Symposium Celerina, August 20-23, 1962, pp
145-157. Copyright © 1963, Springer-Verlag, Berlin.
The Control and Use of Drag-Free Satellites
Benjamin Lange, Doctoral Dissertation, Stanford University Department
of Aeronautics & Astronautics, Paper #194, June, 1964.
The Drag-Free Satellite (Drag free satellite design and use, analyzing
control and guidance system with respect to system performance and gas
usage)
Benjamin Lange. American Institute of Aeronautics & Astronautics
Journal (AIAA), Volume 2, Number 9, Septermber 1964, pp. 1590-1606.
A Satellite Freed of all but Gravitational Forces: "TRIAD I"
Staff of the Space Department, The Johns Hopkins University Applied
Physics Laboratory, Silver Spring, MD AND Staff of the Guidance and
Control Laboratory, Stanford University, Stanford, CA. American
Institute of Aeronautics & Astronautics Journal (AIAA), Volume 11,
Number 9, September 1974, pp. 637-644.
The Stanford Relativity Gyroscope Experiment: History and Overview
C. W. F. Everitt, Excerpt from the book, Near Zero: New Frontiers of
Physics
. Chapter VI, Section 3A. Edited by J.D. Fairbank, B.S Deaver, Jr.,
C.W.F. Everitt, & P.F. Michelson, Copyright © 1988, W. H. Freeman &
Company, New York.
The Stanford Relativity Gyroscope Experiment: Translation and
Orientation Control
Daniel B. De Bra, Excerpt from the book, Near Zero: New Frontiers of
Physics
. Chapter VI, Section 3G. Edited by J.D. Fairbank, B.S Deaver, Jr.,
C.W.F. Everitt, & P.F. Michelson, Copyright © 1988, W. H. Freeman &
Company, New York.
Open sharing of information is crucial to improving everybody's
understanding of the universe around us.
Tom
` pete wrote:
` > In sci.space.policy, on Tue, 31 Oct 2006 07:34:15 -0500,
` > Jeff Findley <jeff.f...@ugs.nojunk.com> sez:
` >
` > ` "Ian Woollard" <ian.wo...@gmail.com> wrote in message
` > ` news:4546ffb0$0$8719$ed26...@ptn-nntp-reader02.plus.net...
` >
` > ` > Bob forward pointed out that you can in principle make a
` > ` > zero-gravity room on the Earth if you could mount a disk of
` > ` > neutronium above it. (The minor implementation details for
` > ` > doing that are left to the reader ;-) )
` >
` > ` Hold it up with scrith? ;-)
` >
` > I think an engineering study would be likely to conclude that a
` > cheaper alternative would be to dig down and construct the
` > room at the centre of the earth... (^:
` Dig down?
` A signifigant mass of neutronium (disregarding how it would be kept
` at that density, once removed from a neutron star [and however that
` removal process would be accomplished]) would likely find its way to
` Earth's core in short order...just, um, follow it.
As the original object was just a room with 0 net gravitational force
on the earth, being at the earth's centre would remove the need
for any neutronium.
> Yep, very clear there.
That is a huge improvement on what the folks at Glenn(/Lewis) used to
say about "microgravity". Here is one webpage that was scrutinized on
this forum a long time ago:
http://microgravity.grc.nasa.gov/
At the time of that discussion (mid-2001) their webpage said this:
"Microgravity literally means a state of very weak gravity
(one-millionth of what is felt on Earth)."
...revealing a complete disconnect with fundamental physics when that
term is applied to the orbital environment.
(Ref -
http://web.archive.org/web/20010517003023/http://microgravity.grc.nasa.gov/)
Not long after this forum highlighted the error, the webpage was
changed to offer this involved explanation:
========
Microgravity is a condition where the effects of gravity appear to be
small or even negligible compared to the normal effects of gravity on
Earth. Where the effects appear to be very small, as on an orbiting
spacecraft, microgravity is generally meant to be synonymous with zero
gravity, zero g, and weightlessness.
The term microgravity can be used to describe a condition where gravity
is actually small, for example on the Moon where gravity is about one
sixth of that on Earth. But we generally use the term microgravity to
describe a condition where gravity is not small, but appears to be
small. This is the condition experienced on orbiting spacecraft, such
as the International Space Station, and all objects in free fall.
The force of gravity diminishes with distance, so gravity is weaker on
the International Space Station (ISS) than on Earth. But ISS is only
about 300 km above the Earth, hardly off the surface on a planetary
scale. At that altitude, the gravitational acceleration is about 90% of
that at the Earth's surface. So gravity on ISS is almost the same as it
is on Earth. And weight is defined as the force of gravity on an
object, so weight on ISS is nearly the same as that on Earth. But given
the images of floating astronauts, it appears that gravity and weight
are reduced by much more than 10%. What's happening?
Gravity causes objects to fall, unless their motion is restricted by
some other force, such as that of the floor. If gravity is the only
force acting on an object's motion, then it is in free fall. Neglecting
the fall itself, free-falling objects behave like there is no gravity.
This happens because free-falling objects experience the same
gravitational acceleration regardless of their mass (in contrast to the
common preconception). Imagine you have an apple on a scale, which
displays the apple's weight. If you drop the scale, the apple and scale
will fall together, but the scale will no longer be compressed by the
apple, so the scale will show zero weight. In the same way, both the
astronauts and ISS are falling towards the Earth. Since they have the
same acceleration, the astronauts seem to have no weight and float
within the ISS. Fortunately, the astronauts and the ISS are moving so
quickly (about 28,000 kph) that they fall around the Earth in a
circular orbit. Similarly, Space Shuttle astronauts experience free
fall while they coast around the Earth (with engines off at 8 minutes
after launch). In both cases, microgravity is achieved because the
spacecraft are in a continuous state of free fall.
While gravity and weight seem to vanish in free fall, gravity-driven
motion like sedimentation and buoyant convection are truly absent. This
allows astronauts to conduct unique experiments that may enable further
space exploration or improvements to our quality of life here on Earth.
========
(Ref -
http://web.archive.org/web/20020524155549/http://microgravity.grc.nasa.gov/)
A major improvement on the previous disconnect. But still glaring
errors were present, such as this:
"The term microgravity can be used to describe a condition where
gravity is actually small, for example on the Moon where gravity is
about one sixth of that on Earth."
...a mistake of several order of magnitude ('one-sixth' versus
'micro-'). And then it contains what I'll call dangerous statements,
that support the ill-conceived term 'microgravity':
"While gravity and weight seem to vanish in free fall, ..."
"If gravity is the only force acting on an object's motion, then it is
in free fall. Neglecting the fall itself, free-falling objects behave
like there is no gravity."
Gravity would "seem to vanish in free fall" only if your mind failed to
grasp the concept that the reason you are in freefall is because of
gravity.
Fast forward to today, the biggest problem I have with that Glenn
webpage:
> http://www.nasa.gov/centers/glenn/research/microgex.html
...is their use of the terms "microgravity" and "zero gravity"
themselves. Everything else looks fairly accurate to me.
So kudos to NASA for marching the path toward accurate physics! Those
are huge steps in a relatively short period of time.
~ CT
Just to be nit-picky, I thought I'd point out that, if the Earth looks
like a point mass and gravity is Newtonian, then the path _is_ a true
parabola _if_ you launch the projectile with just enough energy to escape.
So, the issue isn't really that it's only a parabola for _small_ powder
loads. It's that it's only a parabola for one particular whopping _big_
powder load.
The proof's a bit tedious, and is part of the reason Newton got so
famous :-) . FWIW, here's my version, which I just put up, mostly 'cause I
was so pleased at actually getting through all the details:
http://www.physicsinsights.org/orbit_shapes_1.html
--
Nospam becomes physicsinsights to fix the email
> Just to be nit-picky, I thought I'd point out that, if the Earth looks
> like a point mass and gravity is Newtonian, then the path _is_ a true
> parabola _if_ you launch the projectile with just enough energy to escape.
>
> So, the issue isn't really that it's only a parabola for _small_ powder
> loads. It's that it's only a parabola for one particular whopping _big_
> powder load.
>
> The proof's a bit tedious, and is part of the reason Newton got so
> famous :-) . FWIW, here's my version, which I just put up, mostly 'cause I
> was so pleased at actually getting through all the details:
>
> http://www.physicsinsights.org/orbit_shapes_1.html
Those are some very messy equations you have there on that page. An
extremely clean way to analyze escape vs capture is graphically with an
energy potential well diagram. It's just like those coin wells, except
that it visualizes the energy of the orbiting body in terms of kinetic
energy being the height above the potential well surface. Energy is
conserved so the total (sum of potential + kinetic) stays constant. So
the height of the trajectory doesn't change.
As the spacecraft ventures farther from the primary body, kinetic
energy decreases while potential increases. So the three classes of
orbits become:
- Spacecraft kinetic energy is insufficient to reach the top of the
well (ELLIPTICAL CLASS),
- Spacecraft kinetic energy *exactly* reaches the top of the well
(PARABOLIC CLASS),
- Spacecraft kinetic energy exceeds the top of the well (HYPERBOLIC
CLASS).
The concept can be clearly described with pretty pictures and no messy
equations.
~ CT
Then when a massive body is introduced into this space, the sheet
deforms and produces a gravity well. Notice that nowhere on the sheet
will you find zero gravity any more for any finite distance from this
mass.
So the energy well has stretched below the zero point. Any satellite
with negative total energy will be captured by this body. But a
thrusting maneuver can increase kinetic energy, raising its energy,
depicted as height above the rubber sheet. If that kinetic energy
increases to the height of the original undeformed sheet (the point of
zero total energy) then you have a parabolic orbit. If the kinetic
energy increases to the point of excess total energy (positive) then
you have a hyperbolic trajectory.
~ CT
>>From sal:
>
>> Just to be nit-picky, I thought I'd point out that, if the Earth looks
>> like a point mass and gravity is Newtonian, then the path _is_ a true
>> parabola _if_ you launch the projectile with just enough energy to
>> escape.
>>
>> So, the issue isn't really that it's only a parabola for _small_ powder
>> loads. It's that it's only a parabola for one particular whopping _big_
>> powder load.
>>
>> The proof's a bit tedious, and is part of the reason Newton got so
>> famous :-) . FWIW, here's my version, which I just put up, mostly 'cause
>> I was so pleased at actually getting through all the details:
>>
>> http://www.physicsinsights.org/orbit_shapes_1.html
>
> Those are some very messy equations you have there on that page. An
> extremely clean way to analyze escape vs capture is graphically with an
> energy potential well diagram.
Oh, absolutely. That's the way one normally does it, for sure; you are
correct.
However, the point of the page was to actually solve the equations to find
the orbit shapes, not just the escape velocity.
In particular, you can easily find the escape velocity with an energy
argument -- but can you prove the trajectory at escape velocity is a
parabola that way, and an ellipse if you miss escaping but have too much
energy for a circle? AFAIK that proof is a mess no matter how you tackle
it -- but I'd be glad to learn otherwise.
> It's just like those coin wells, except
> that it visualizes the energy of the orbiting body in terms of kinetic
> energy being the height above the potential well surface. Energy is
> conserved so the total (sum of potential + kinetic) stays constant. So
> the height of the trajectory doesn't change.
>
> As the spacecraft ventures farther from the primary body, kinetic energy
> decreases while potential increases. So the three classes of orbits
> become:
>
> - Spacecraft kinetic energy is insufficient to reach the top of the well
> (ELLIPTICAL CLASS),
But how do you prove it's really an ellipse, without the mess?
> - Spacecraft kinetic energy *exactly* reaches the top of the well
> (PARABOLIC CLASS),
How do you prove it's a parabola, without the mess of solving the
equations and verifying the form of the result?
> - Spacecraft kinetic energy exceeds the top of the well (HYPERBOLIC
> CLASS).
And again, how do you prove it's really a hyperbola, short of solving the
equations of motion?
>
> The concept can be clearly described with pretty pictures and no messy
> equations.
>
>
> ~ CT
--
Nospam becomes physicsinsights to fix the email
I can be also contacted through http://www.physicsinsights.org
For the parabolic case, the potential well diagram shows that the
farther the spacecraft gets from the primary body, the closer its
kinetic energy gets to zero. So the farther it gets, the slower it
gets. The ramification of this trend toward zero velocity as it
approaches infinite distance is that the trajectory will also take
infinite time.
...of course, this analysis is only two body theory. An extremely
clinical version of the universe. There are many other factors as to
what actually happens to spacecraft like the Voyagers and such.
~ CT
~ CT
Agreed. I'm not aware of any way to derive the trajectory geometry
from the energy analysis. Thanks for clearing up for me what you're
accomplishing with those detailed equations.
~ CT
> I can be also contacted through http://www.physicsinsights.org
Hey, sal. Out of curiosity I poked around your website a bit. Looks
like some neat stuff there. If you're open to constructive feedback, I
saw that on this page:
http://www.physicsinsights.org/lagrange_1.html
...you've got Newton's Second Law as F=ma.
With as exacting as you have shown to be, I would expect that you'd
cringe as much as I do whenever that kind of oversimplification to the
point of inaccuracy is done. It is cited about as commonly as the term
"zero gravity" is used, but hardly as egregious. Heh.
~ CT
>Oh, absolutely. That's the way one normally does it, for sure; you are
>correct.
...Saf, just a word of strong advice: "Stuff4" or ~CT, is a known
troll who's plagued this newsgroup for going on six years now. His
standard tactic is to ask a seemingly innocent, inoffensive question,
then twist the topic into a flamefest. If you do a Google Groups
search, you'll find dozens of people who've been suckered by his game,
most of whom get wise a bit too late for the rest of us, but still
eventually killfile him and put him out of our misery. That is, until
the next sucker comes along, and...well, I hate to say it, but you're
the current next sucker.
Please, just killfile him and be done with him. We'll appreciate you
for it.
OM
--
]=====================================[
] OMBlog - http://www.io.com/~o_m/omworld [
] Let's face it: Sometimes you *need* [
] an obnoxious opinion in your day! [
]=====================================[
Errr .... I wrote down Newton's laws on that page from memory, didn't
double check 'em. Time for a big "Ooops", maybe?
Gotta run off right now but I'll check that out later today. Tx.
>
>
> ~ CT
--
Nospam becomes physicsinsights to fix the email
There are those who appreciate having it pointed out that Newton's
Second Law is *not*:
F = ma
(or F = m dv/dt). There are those who appreciate many of the other
fallacies I have exposed throughout these years. Even this very thread
on "microgravity" exposes a horribly erroneous misunderstanding of
orbital physics that has permeated popular understanding, as well as
the "experts" at NASA themselves. We could review every single thread
that I've posted to here that has degenerated into a "flamefest" and I
see the consistent theme that I present the view that I see as accurate
history, accurate physics, etc. Anger and hatred spawn from the
unwillingness of others to so much as consider the point of view that
has been offered. You'd also see that the anger and hatred is only
present on one side of such discourse. I also consider it important to
note that none of the views I've presented are offered as TRUTH. They
are offered as my best understanding, where I remain open to the
possibility that my beliefs and views are in need of improvement.
For one more case study, consider the current thread about Apollo 13.
Here we have the amazing opportunity to ask questions of one of the
primary players. The thread itself is titled "Questions about Apollo
13". I have been extremely patient throughout the development of that
thread in questioning the notion that the O2 tank rupture was an
"explosion", a view that clearly was not promoted by the official
investigation. The ramification of this factual understanding is that
the Direct Abort option was more viable than NASA and Ron Howard would
have us believe. This means that the decision to continue around the
Moon may have been an unnecessary risk of killing off the astronauts
due to the consumables constraints.
This forum claims to be about history. I am here to discuss accurate
history, for the purpose of learning from the past. If we cannot get
so much as the straight story on Apollo 13, then we are not improving
the probability of getting CEV astronauts home from the Moon in one
piece.
These are the basic reasons why I persist in posting my views here in
spite of how much hatred is voiced against me. I also take comfort in
knowing that accurate facts will prevail, if given enough time. It is
easy to show that
F = ma
is inaccurate under certain conditions. For instance, you cannot
derive the rocket equation from it. You must start with the actual 2nd
Law. Likewise I am confident that some future generation will get a
good laugh at how the terms "zero gravity" and "microgravity" are used
today.
How is it that I post to ssh, and the vocal majority attacks me. I
post the very arguments to Wikipedia and they are embraced? By this
very forum even!
That is a clear sign to me that this forum is broken.
~ CT
>>From sal:
>> On Sun, 19 Nov 2006 21:19:09 -0800, Stuf4 wrote:
>>
>> >>From sal:
>> >
>> >> Just to be nit-picky,
> <snip>
>
>> I can be also contacted through http://www.physicsinsights.org
>
> Hey, sal. Out of curiosity I poked around your website a bit. Looks like
> some neat stuff there. If you're open to constructive feedback, I saw
> that on this page:
> http://www.physicsinsights.org/lagrange_1.html
>
> ...you've got Newton's Second Law as F=ma.
Ah, hmm... yeah, I looked that over again. I should rewrite that section.
The problem is one of definition, of course. Newton's original, though
written in Latin rather than calculus, translates pretty directly into
f=ma, but, of course, the more useful form is f=dP/dt since it
generalizes to nonconstant mass. The problem with the latter is how we
define P.
The definition which is probably closest to correct is "That vector
quantity which is conserved and which behaves like mv in simple cases" but
that's hard to work with.
OTOH if it's defined simply as "mv", and we take m to be constant (for
Newtonian mechanics), then we get back to f=ma in one step, and that's
probably what I should say on that page.
If we _define_ it as "@T/@v" then we've skipped a lot of steps -- that
doesn't seem quite right for the initial definition of P.
Anyway, upon looking back at that page I think it could use a little more
explanation between equations (4) and (5) and I also think
you're right, I should restate the second law.
> With as exacting as you have shown to be, I would expect that you'd
> cringe as much as I do whenever that kind of oversimplification to the
> point of inaccuracy is done. It is cited about as commonly as the term
> "zero gravity" is used, but hardly as egregious. Heh.
>
>
> ~ CT
--
Nospam becomes physicsinsights to fix the email
For documentation, here is that Wikipedia entry on Weightlessness in
its entirety:
===========
Criticism of the terms "Zero Gravity" and "Microgravity"
It is important to note, as stated at the beginning of this article,
that there is plenty of gravity pulling on a spacecraft in orbit around
the Earth. Gravity is the very reason why the spacecraft is orbiting.
Therefore it is totally inaccurate to say that astronauts are
experiencing "zero gravity" or "microgravity". What orbiting astronauts
experience is zero-g, a measure of acceleration relative to their
spacecraft, which results in weightlessness. But zero-g is not "zero
gravity". If there were "zero gravity" or "microgravity", their
spacecraft would not be pulled into an orbit around the Earth. It would
go in a straight line.
As a thought experiment, imagine a spacecraft that had the ability to
rise up to orbital altitude by going straight up like a helicopter and
hover over one spot on the Earth. The astronauts inside would not
experience weightlessness. Their ride inside this hovering spacecraft
would be similar to riding an elevator up an incredibly tall building
and stopping at the top floor. While hovering above Earth's atmosphere,
their weight would be very close to what they weigh on the surface of
the Earth, even as a space shuttle goes zinging by them. So astronauts
in a hovering spacecraft are being pulled by strong gravity just as
space shuttle astronauts are pulled by strong gravity. The difference
between them is that the orbiting shuttle is freely being pulled toward
the center of the Earth. The lack of relative acceleration between the
orbiting shuttle and its astronauts inside (who are also being freely
pulled toward the center of the Earth) result in them being weightless.
But the hovering spacecraft (as with an elevator at the top of an
incredibly tall building) is not freely falling. The pull of gravity it
is experiencing is being opposed by the hovering force. This force gets
transfered to the astronauts within (along with everything else within
the spacecraft) resulting in weight. This example illustrates the fact
that there is plenty of gravity out in space. If you were to take any
object that is orbiting the Earth and stop it dead in its track and
then release it, the Earth's gravity would pull it straight down back
toward the Earth's surface.
To use confused terms like "zero gravity" and "microgravity" is to
mistake the general concept of acceleration for the concept of gravity.
"Zero-g" and "micro-g" are perfectly accurate terms referring to the
lack of acceleration (in the frame of reference of the spacecraft) that
cause weightlessness, even while gravity is strongly pulling the
trajectory of that spacecraft into an orbit.
The specific point of confusion is that "g" does not mean "gravity".
The designator "g" is an arbitrary scale of acceleration not to be
confused with gravity itself. "Zero-g" means zero acceleration, not
zero gravity. "1-g" is the acceleration experienced on the surface of
the Earth due to gravity, but it is not gravity itself. This scale is
widely used because it is easy to relate to from common experience of
acceleration due to gravity. But any other scale of acceleration can be
used to describe the condition of weightlessness. It could be described
using a scale that has nothing to do with Earth's gravity. Similarly, a
distance can be measured in feet as well as meters, where a meter has
nothing to do with the length of a human foot. For a weightless
astronaut to say that they are in zero gravity is the same type of
error as saying that an object that has a length of 0.3048 meters is
identically one human foot in flesh and blood. "1 foot" is an arbitrary
scale for measuring length that was (at some point in history) based on
a person's foot, but not to be confused with an actual human foot.
"1-g" is an arbitrary scale for measuring acceleration that is based on
gravity, but not to be confused with actual gravity. A zero-g
environment is also a zero-meters/second^2 environment and a
zero-feet/second^2 environment. Any arbitrary scale of acceleration can
be used, and none of them have any exclusive relationship to gravity.
Another illustration of this type of mistake is when people erroneously
speak of a jet pilot blacking out as a result of "gravity-induced Loss
Of Consciousness". The proper term is g-induced Loss Of Consciousness.
It is the acceleration produced by their maneuvers that is the culprit
for g-LOC. It is clearly "g-induced" and not "gravity-induced", because
gravity obviously remained constant at 1-g the whole time for the
pilot. Likewise, the purpose of NASA's " Reduced Gravity Aircraft" is
not to reduce gravity, but rather to fly in a parabolic arc that brings
relative acceleration to zero. "g" is reduced while gravity stays
essentially the same. So clearly it is possible to experience zero-g
without going into space. Any aircraft can do this by pushing it over
into a parabolic arc. Even any car that hits a bump fast enough to
leave the ground will experience zero-g for the time that the wheels
are not in contact with the road. The easiest way to experience zero-g
is to bend your legs and jump off the ground. For the time that you are
in the air, you are experiencing weightlessness. The difference with
astronauts is that the experience is not momentary because their
spacecraft is continually getting pulled toward the Earth. It is
possible for non-astronauts to experience longer durations of
weightlessness by cliff diving, bungee jumping, freefall parachuting,
barrelling over a waterfalls or more safely by riding many types of
modern amusement park rides that put the occupant in a freefall. What
is common for the astronauts as well as these other examples is that it
is not gravity that is changing, but rather the acceleration in their
falling frame of reference goes to zero-g.
As it stands today, NASA itself is one of the biggest promoters of the
erroneous terms "zero gravity" and "microgravity" (along with the
similarly erroneous term "reduced gravity"). Astronauts themselves have
been quoted as having experienced "no gravity" while in space. Surely
they are aware that there was plenty of gravity throughout every orbit
they made, with gravity being the very thing that pulled them into an
orbit.
===========
An advantage that Usenet has over Wikipedia is that it is easy to
search for the exact words that a person has posted. This is the most
durable means of sharing information that I know of.
...and that's another reason why I don't use nasty language here, no
matter how much I get "flamed". If any of my arguments turn out to be
deficient in logic, I'm ok with having made a factual mistake. But I
strive to maintain civility because I post with an awareness that
great-great-grandkids will have direct access to your character.
~ CT
> > Hey, sal. Out of curiosity I poked around your website a bit. Looks like
> > some neat stuff there. If you're open to constructive feedback, I saw
> > that on this page:
> > http://www.physicsinsights.org/lagrange_1.html
> >
> > ...you've got Newton's Second Law as F=ma.
>
> Ah, hmm... yeah, I looked that over again. I should rewrite that section.
>
> The problem is one of definition, of course. Newton's original, though
> written in Latin rather than calculus, translates pretty directly into
> f=ma, but, of course, the more useful form is f=dP/dt since it
> generalizes to nonconstant mass. The problem with the latter is how we
> define P.
>
> The definition which is probably closest to correct is "That vector
> quantity which is conserved and which behaves like mv in simple cases" but
> that's hard to work with.
>
> OTOH if it's defined simply as "mv", and we take m to be constant (for
> Newtonian mechanics), then we get back to f=ma in one step, and that's
> probably what I should say on that page.
>
> If we _define_ it as "@T/@v" then we've skipped a lot of steps -- that
> doesn't seem quite right for the initial definition of P.
>
> Anyway, upon looking back at that page I think it could use a little more
> explanation between equations (4) and (5) and I also think
> you're right, I should restate the second law.
The irony I saw there is that you went from F=ma and propogated it into
the 2nd Law! I imagined your reason for doing so is that the majority
of your audience would not have recognized it as the 2nd Law if you had
simply stated it as:
_
F=dp/dt
Heh!
(And to reiterate what I've responded to OM, this distinction is
especially important on this forum because you can't get to the rocket
equation from F=ma.)
~ CT
F = d(mv)/dt (rate of change of momentum)
If m = constant, F = ma.
- - -
At this point, I would respectfully request that you cease encouraging
CT to begin spinning his net once again.
Thanks.
--
Dave Michelson
da...@ece.ubc.ca
Dave Michelson wrote:
>
> At this point, I would respectfully request that you cease encouraging
> CT to begin spinning his net once again.
I still remember the time I actually agreed with something CT wrote.
He replied that I was wrong. :-D
Pat
>>From sal:
>> On Mon, 20 Nov 2006 00:07:36 -0800, Stuf4 wrote:
> The irony I saw there is that you went from F=ma and propogated it into
> the 2nd Law!
Um, yeah, so I did...
> I imagined your reason for doing so is that the majority of
> your audience would not have recognized it as the 2nd Law if you had
> simply stated it as:
> _
> F=dp/dt
>
> Heh!
>
> (And to reiterate what I've responded to OM, this distinction is
> especially important on this forum because you can't get to the rocket
> equation from F=ma.)
You mean this one?
Assume an infinitesimal parcel of fuel weighs dm, and, including the
parcel, the rocket weighs m+dm; after ejecting the parcel it will
weigh m. If exhaust velocity is v_e, then by conservation of sum(mv):
(m + dm)v = m(v + dv) + dm(v - v_e)
Canceling similar terms,
0 = m dv - v_e dm
Multiplying through by 1/(m dm) and rearranging,
dv/dm = v_e/m
Integrating,
delta v = v_e log(m_i/m_f)
Since conservation of sum(mv) can be shown starting from F=ma and
F1=-F2, and conservation of sum(mv) is all I used (along with some
sloppy infinitesimals), I think that might qualify as doing it from
F=ma rather than F=dp/dt. :-)
It's when you can change the mass of a body without splitting off a second
body or merging a second body into it, as you can in relativity, that you
really run into trouble with F=ma.
Wow, I'd never seen that one before. Nice. Looks like I was mistaken
about the "can't" part. At first glance it seems to me that you are
skipping the mass variability ramifications of dp/dt and just starting
with the (m + dm) stuff as the baseline.
~ CT
Yeah. We wouldn't want to encourage anyone in this forum to go about
correcting errors of fact.
...like what Newton's 2nd Law is and isn't. ...like what gravity is
and isn't. ...like what Apollo 13 abort options were and weren't.
Etc.
~ CT
It's amazing what weirdoes shop at Wal-Mart. ;-)
Jeff
--
"They that can give up essential liberty to obtain a
little temporary safety deserve neither liberty nor
safety"
- B. Franklin, Bartlett's Familiar Quotations (1919)
>
>"Stuf4" <tdadamemd-...@excite.com> wrote in message
>news:1164008443.2...@j44g2000cwa.googlegroups.com...
>> By the way, yesterday I was out at Walmart with Joe Engle, in case
>> anyone was interested.
>
>It's amazing what weirdoes shop at Wal-Mart. ;-)
I'm trying to figure out why Stuffie thinks anyone would be
interested.
> I still remember the time I actually agreed with something CT wrote.
> He replied that I was wrong. :-D
It happens quite often that agreement is voiced with a view that is
presented after having misunderstood what was said.
For an extreme example...
A flock can follow a shepherd. Doesn't necessarily means that each
sheep comprehends the reasons for the path taken.
~ CT
> >"Stuf4" <tdadamemd-...@excite.com> wrote in message
> >news:1164008443.2...@j44g2000cwa.googlegroups.com...
> >> By the way, yesterday I was out at Walmart with Joe Engle, in case
> >> anyone was interested.
> >
> >It's amazing what weirdoes shop at Wal-Mart. ;-)
>
> I'm trying to figure out why Stuffie thinks anyone would be
> interested.
Well, when one observes someone like Sy Liebergot make a statement
about Apollo 13 that "direct abort was not an option", and that goes
unchallenged for a whole month, one might conclude that people here
don't care that much about accurate Space History, and the people like
Joe Engle who helped make it.
(http://groups.google.com/group/sci.space.history/msg/2d710f1b40d808b7?dmode=source&hl=en)
~ CT
>>From Rand Simberg:
>> "Jeff Findley" <jeff.f...@ugs.nojunk.com>:
>
>> >"Stuf4" <tdadamemd-...@excite.com> wrote in message
>> >news:1164008443.2...@j44g2000cwa.googlegroups.com...
>> >> By the way, yesterday I was out at Walmart with Joe Engle, in case
>> >> anyone was interested.
>> >
>> >It's amazing what weirdoes shop at Wal-Mart. ;-)
>>
>> I'm trying to figure out why Stuffie thinks anyone would be
>> interested.
>
>Well, when one observes someone like Sy Liebergot make a statement
>about Apollo 13 that "direct abort was not an option", and that goes
>unchallenged for a whole month, one might conclude that people here
>don't care that much about accurate Space History, and the people like
>Joe Engle who helped make it.
Which continues to be irrelevant to whether or not you went to Walmart
with him, and of no obvious interest to anyone.
Rand Simberg wrote:
>I'm trying to figure out why Stuffie thinks anyone would be
>interested.
>
>
I'm trying to figure out why Stuffie thinks anyone would believe him.
Just a week ago, when I was out at K Mart with Osama bin Laden... :-)
Pat
Someone has to old the bag...
>
> Pat
Someone has to hold the bag...
>
> Pat