microgravity - I stand corrected

Why reading the following two pages:

http://www.nasa.gov/centers/glenn/re.../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

"Eric Chomko" wrote in message
oups.com...
Why reading the following two pages:

http://www.nasa.gov/centers/glenn/re.../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.

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)

On Mon, 30 Oct 2006 16:38:28 -0500, in a place far, far away, "Jeff
Findley" made the phosphor on my monitor
glow in such a way as to indicate that:


"Eric Chomko" wrote in message
roups.com...
Why reading the following two pages:

http://www.nasa.gov/centers/glenn/re.../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.

"Rand Simberg" wrote in message
...
On Mon, 30 Oct 2006 16:38:28 -0500, in a place far, far away, "Jeff
Findley" made the phosphor on my monitor
glow in such a way as to indicate that:
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/revi...ion/ncanon.htm

And an interactive web page with animation for the video game generation:

http://www.waowen.screaming.net/revi...n/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".

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)

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)

That's why it is possible for an observer in
a sealed room under a 1-g field to differentiate
between mass-induced weight, thrusting
weight, and centrifugal (end-of-tether) weight.

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.

: "Jim Oberg"
: That's why it is possible for an observer in a sealed room under a 1-g
: field to differentiate between mass-induced weight, thrusting weight,
: and centrifugal (end-of-tether) weight. 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 http://sheol.org/throopw

Jim Oberg wrote:
That's why it is possible for an observer in
a sealed room under a 1-g field to differentiate
between mass-induced weight, thrusting
weight, and centrifugal (end-of-tether) weight.

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.

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 ;-) )

Rand Simberg wrote:
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.

Real gravitational fields are never actually point source equivalents,
so it's never actually a partial ellipse.

On Mon, 30 Oct 2006 20:44:12 +0000, Rand Simberg wrote:

On Mon, 30 Oct 2006 16:38:28 -0500, in a place far, far away, "Jeff
Findley" made the phosphor on my monitor
glow in such a way as to indicate that:


"Eric Chomko" wrote in message
groups.com...
Why reading the following two pages:

http://www.nasa.gov/centers/glenn/re.../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.

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

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. 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