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Grex > Agora35 > #20: Movies, movies, movies, movies, movies, movies, movies | |
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| 25 new of 323 responses total. |
rcurl
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response 193 of 323:
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Nov 13 17:34 UTC 2000 |
How else can an object enter (change) orbit except by accelerating?
Acceleration is a rate of change of velocity and, boy, if you don't
change velocity, you are stardust (or miss).
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tpryan
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response 194 of 323:
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Nov 13 17:41 UTC 2000 |
To be captured into an orbit, you have to decelerate, that is,
provide thrust against the direction of your travel, otherwise, you
could continue past the planet.
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rcurl
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response 195 of 323:
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Nov 13 18:29 UTC 2000 |
That's an acceleration too. If you are facing backwards, you will
feel it as a subjectively forward acceleration. If you insist on
referencing to the velocity vector, yes, it is a *negative* acceleration.
(I'm being picky.... 8^P)
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gelinas
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response 196 of 323:
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Nov 13 21:03 UTC 2000 |
_Red_Planet_ was one of Heinlein's books. I remember reading it, but I
*don't* remember the plot. I should track it down.
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mwg
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response 197 of 323:
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Nov 13 21:22 UTC 2000 |
I was at a friends the other night re-watching _Xanadu_ for the zillionth
time, and finally caught a very subtle joke that I had missed for years.
Sonny is confronting the parents of the Muses asking for Terpsichore to be
allowed to return to Earth with him, and the mother is having difficulty
remembering such concepts as earlier/later one-night/forever.
Before I post an answer, does anyone else know why this is funny, and/or
how I worked out the correct name of the Muse in question?
(Obscure mode on.)
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aruba
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response 198 of 323:
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Nov 13 23:15 UTC 2000 |
Red Planet is my very favorite Heinlein book of all, and I am disappointed
that someone used the title for a completely unrelated movie.
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mcnally
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response 199 of 323:
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Nov 14 02:16 UTC 2000 |
re #197: I've never seen the movie, so I'm not sure why Terpsichore
(although I imagine it has something to do with dance, isn't the movie
about a roller-disco?)
Presumably the "joke" with the mother is because the traditional depiction
of the Muses has them the offspring of Zeus and Mnemosyne (aka "Memory")
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mwg
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response 200 of 323:
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Nov 14 14:16 UTC 2000 |
Re: 199 - Exactly. On previous occasions I had not known the name of the
mother of the Muses, so this time the scene hit me differently. Olivia
Newton-Johns' character was called Kira (maybe not spelled quite that way)
and at one point she is breaking the news to Sonny and gets out a partial
syllable on her name before Sonny muffles the rest with a kiss. I looked
up the names of the Muses and found them to be unique in less than a
syllable, so the "te" that got out in the described scene made Terpsichore
the only option. And yes, it was a roller-disco-dance-musical, so the
Muse of dance fits. They layered in the jokes heavily, even though most
of the audience would never get them. They deserve credit for that, at
least.
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bdh3
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response 201 of 323:
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Nov 15 06:23 UTC 2000 |
re#192,194,195: One enters orbit around a planet by attaining orbital
velocity. I suppose if you were going real slow if you didn't
accelerate you'd simply be captured by the gravitational acceleration of
the object you were near and be accelerated onto the surface of that
object. But that is usually not the goal of the exercise. If one is
traveling between two objects, and one wishes to do so in an optimum
time/resources, one accelerates towards the object, turns over and then
'burns' a deceleration such that the 'gravitational acceleration'
captures you into orbit. Subjectively if one were on the object
decelerating one might feel like one was accelerating in the opposite
direction regardless of the fact that velocity was decreasing. Perhaps
one could have 'main engines' on both ends of a vehicle, but that would
be rather stupid. My point remains, I don't think there is a film -
exception maybe _2001..._ where the craft enters orbit properly ass
backwards (Even in that the jupiter craft was always pointing forward).
(I believe even _Apollo 13_ made this mistake although I am not certian
and that was not an attempt to orbit but to 'slingshot around'.)
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senna
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response 202 of 323:
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Nov 15 07:30 UTC 2000 |
Okay, layperson question: Would it be reasonable to assume that if one were
to somehow accelerate to the velocity of a certain orbit from near the
surface, they would continue away from the planetary body until reaching that
orbit, and then automatically self-correct?
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bdh3
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response 203 of 323:
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Nov 15 09:46 UTC 2000 |
No.No. No. No. And yes. If one were originally doomed to smash into a
'planetary body' and then accelerate so that one obtained orbit one
would merely orbit said 'body' until such time as the orbit 'decayed'
and one smashed into the surface if one did not burn up if there was
sufficient 'atmosphere'. There is no 'automatic'. It is all pure math.
THere are no 'votes', there are no 'opinions', it is all 'fact'. IT is
all 'math'. And the fact remains that any idiot that portrays an
accelleration into orbit is an idiot, and probably a fine
'cinematographer'. Its a good 'shot', has nothing to do with 'real'
science, and is probably what one should shoot.
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senna
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response 204 of 323:
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Nov 15 12:22 UTC 2000 |
Okay, let me rephrase: Starting from the surface of Earth (or, better yet,
some atmosphereless Earth-sized rock) I immediately accelerate to a speed
which would maintain an orbit of, say, 150 miles, initially in a tangent
course to the curvature of the celestial body. What happens, barring
unforseen variables?
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flem
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response 205 of 323:
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Nov 15 14:57 UTC 2000 |
As I understand (and I'm no rocket scientist either), an orbital velocity is
just an equilibrium, where the tendency to fall (due to gravity) and the
tendency to shoot away from the, say, planet (due to inertia) cancel each
other out. Because of the way gravity works (inverse square law), the speed
necessary for orbital velocity depends on how far you are from the planet.
So, the speed necessary to orbit the earth at ten feet off the ground is
higher (gravity has more effect) than the speed necessary to orbit at 100
miles altitude.
So, your question. If you accelerate to whatever speed would be required
to maintain orbital velocity at 150 miles altitude, then turn off your
engines, it would take time for you to ascend. During that time, gravity
would accelerate you towards the earth, changing your velocity considerably.
If you didn't turn your engines back on, you would certainly leave a
very large crater.
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janc
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response 206 of 323:
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Nov 15 15:20 UTC 2000 |
A brief non-mathematical (and I think pretty intuitive) explanation of why
things stay in orbit is at http://www.wwnet.net/~janc/bestthings/square.htm
l
Another nice paradox: You are in orbit around the earth. Another space ship
is in the same circular orbit, ahead of you by 1/8 of a circumference. You
wish to catch up with the other ship. To do so should you (1) speed up or (2)
slow down?
The correct answer is to slow down.
If fire a blast from your engines to speed you up, you will start moving away
from the earth in an elliptical orbit with it's apogee (nearest point to earth)
at the point where you fired your engine). This is a much longer orbit,
and you will wind up falling behind your target. Slowing down drops you
into a lower, faster orbit. As long as you don't re-enter (oops) you should
be able to catch up with your target on that orbit.
Visualize the model of a marble rolling around in a funnel. Speeding the
marble up makes it go higher in the funnel, slowing down as it climbs, and
having a longer path to travel, slowing down makes you dip lower, speeding
up as you do so, and taking a shorter path.
Basic rule is that you don't fly a space ship by pointing it where you want
to go and hitting the gas, not unless you have really hellaciously large
amounts of fuel to burn and don't care how fast you are going when you hit
your target.
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albaugh
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response 207 of 323:
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Nov 15 17:10 UTC 2000 |
That scenario assumes that you can't alter your tragetory as you accelerate.
If you "point down" while speeding up, you can certainly close on your target.
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other
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response 208 of 323:
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Nov 15 21:31 UTC 2000 |
"tragetory"...? s/get/ject
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albaugh
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response 209 of 323:
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Nov 15 21:56 UTC 2000 |
target + ory ?
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drew
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response 210 of 323:
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Nov 15 22:37 UTC 2000 |
On #205: If you accelerate from ground but don't circularize, you're in an
elliptical orbit with apogee at whatever altitude you reach and perigee at
ground level. Doing this from the Moon should be safe so long as there are
no mountains in the way. From any world with an atmosphere it's a slightly
different matter: in practice it's necessary to clear most of that atmosphere
which doing so puts you near if not at whatever altitude will suffice for a
low orbit.
Boosting forward or aft at orbit apogee raises or lowers, respectively, the
perigee, and vice versa.
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drew
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response 211 of 323:
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Nov 15 22:41 UTC 2000 |
Oh yes. And while I'm certain that russ shares the point of view, the
complaint about ships being pointed in the wrong direction half the time in
space movies/TV shows is most of the time mine.
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brighn
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response 212 of 323:
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Nov 15 23:51 UTC 2000 |
Saw BWII: Book of Shadows, and was pleased to learn that, in my case at least,
the inverse of the movie reviews also held. The movie's generic review has
been, "If you're going in expecting Blair Witch again, because you loved it,
you'll be disappointed." I thought Blair Witch was the biggest waste of
celluloid ever produced, and only went to BoS because (a) the teasers looked
good and (b) it was so thoroughly panned. It was very good, especially for
its genre.
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rcurl
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response 213 of 323:
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Nov 16 05:42 UTC 2000 |
Space ships in movies and TV (as well as the occupants) are almost always
oriented with respect to "up" and "down" as we experience it on the
earth's surface. I guess they don't want to make the viewers nauseous...
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ric
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response 214 of 323:
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Nov 16 18:14 UTC 2000 |
(The camera is upside down too, actually, so everything just looks right side
up :)
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rcurl
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response 215 of 323:
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Nov 16 22:07 UTC 2000 |
When a bunch of ships gather (such as, to come to another ships rescue, etc),
they are all right-side-up (oh, maybe a few are tilted this way and
that).
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tpryan
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response 216 of 323:
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Nov 16 23:12 UTC 2000 |
In Wrath of Kahn, Spock mentions how Kahn is thinking in only
two dimensions, and has the ship go relative down. He then came up
to shot at the thin profile of the oppenent. All that had to be done
when hidden, was to also turn 90 degrees, to take aim at a big broad
, easy to hit saucer section.
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senna
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response 217 of 323:
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Nov 17 02:36 UTC 2000 |
Indeed, his mention of two dimensions only served to emphasize how
two-dimensional space combat was in the Star Trek universe. Still is, really.
Note that instead of simply nosing down, the Enterprise elected to change
elevation while remaining level, as if on an elevator. No sense in that.
Return of the Jedi probably has the best space combat scenes that never
attempt to maintain a level playing field.
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