0681: "Gravity Wells"

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sje46
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Re: "Gravity Wells" Discussion

Postby sje46 » Mon Dec 28, 2009 7:36 am UTC

Ferahgo wrote:
sje46 wrote:The things making noise on Titan...is that a Vonnegut reference?


That was my thought also. If it is, then well done, as that's one of my favorite books ever.

One of mine as well. Amazing science-fiction. This is what I'm talking about, if anyone's wondering: http://en.wikipedia.org/wiki/Sirens_of_titan
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Re: "Gravity Wells" Discussion

Postby skeptical scientist » Mon Dec 28, 2009 7:37 am UTC

Also, does anyone know what Randall means by, "The planet sizes are to the same scale as the wells"? Jupiter and Saturn being the same size suggests that the sizes are not scaled according to mass or well depth, but the relative size of Jupiter and the Earth suggests that the planets are not scaled according to radius either, so I'm a bit puzzled.
Last edited by skeptical scientist on Mon Dec 28, 2009 7:40 am UTC, edited 1 time in total.
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Re: "Gravity Wells" Discussion

Postby westrim » Mon Dec 28, 2009 7:38 am UTC

Someone needs to hurry up and integrate the threads. But I'll concede defeat (this time, Gadget!) and kill mine.

Oh no, the comic was late by an hour and 20 minutes! HELL IS FREEZING OVERRRRR!
Well, I hope Randall had a good Christmas, though I think this is a pretty liberal interpretation of the words "a 'little' late".
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Re: "Gravity Wells" Discussion

Postby skeptical scientist » Mon Dec 28, 2009 7:41 am UTC

westrim wrote:Someone needs to hurry up and integrate the threads.

Integrating the threads is not in the moderators' job descriptions, so far as I know, but replying only to the first thread posted is in yours.
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Re: "Gravity Wells" Discussion

Postby Interactive Civilian » Mon Dec 28, 2009 7:42 am UTC

Add me to the growing crowd of people saying "Excellent Vonnegut reference." :D

That was the first Kurt Vonnegut book I'd ever read.
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Re: "Gravity Wells" Discussion

Postby Nereid » Mon Dec 28, 2009 7:52 am UTC

A perhaps simpler (and conceptually easier to work with), equivalent formula for well depth is (mass of body / mass of Earth) * (radius of Earth). From this it is obvious that the Earth's well depth should be equal to its radius.

[Remember that g = G * (mass of Earth) / (radius of Earth)^2.]
Last edited by Nereid on Mon Dec 28, 2009 7:54 am UTC, edited 1 time in total.

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Re: "Gravity Wells" Discussion

Postby hogri » Mon Dec 28, 2009 7:53 am UTC

Physical question: Does the absolute height (in the comic) of a planet's baseline or their well's top point respectively have any physical meaning?

I.e. Mercury's well is very small but Venus's well lies much more higher creating a very steep edge between both planets. This could just be in terms of depiction (e.g. you have to make these steep edges so that Jupiter's well does fit into the image).

If there is a meaning, what does it mean if I just escaped Mercury's gravitational force (i.e. I'm on the top edge of it's well) and I want to go to Venus? Do I have to consume much more energy to get to the top of the steep edge and the top of Venus's well?

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Re: "Gravity Wells" Discussion

Postby Nereid » Mon Dec 28, 2009 7:56 am UTC

Yes to both of your yes/no questions.

I wonder if the top of the image represents the height needed to escape the Sun's gravitational well (i.e. the solar system)? It looks about right.

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Re: "Gravity Wells" Discussion

Postby phillipsjk » Mon Dec 28, 2009 8:06 am UTC

Nereid wrote:Earth is a celestial body, but not an astronomical body. http://en.wikipedia.org/wiki/Astronomical_object


I admit I am sometimes wrong about things, so checked the link you provided:

Wikipedia wrote:The terms astronomical objects and astronomical bodies differ from celestial objects and celestial bodies only in that the latter terms do not include the Earth.


so the terms "celestial objects" and "celestial bodies" (literally referring to objects in the sky) . . . do not include the Earth. 8)

Now, I am on more shaky ground with my assertions about the Doomed Space Marine. But, that is a little off topic anyway.

skeptical scientist wrote:Also, does anyone know what Randall means by, "The planet sizes are to the same scale as the wells"? Jupiter and Saturn being the same size suggests that the sizes are not scaled according to mass or well depth, but the relative size of Jupiter and the Earth suggests that the planets are not scaled according to radius either, so I'm a bit puzzled.


Maybe the scales are logarithmic. (Randal has pulled that trick with past "posters")
(he does explicitly measure the gravity wells in km: a distance measurement. (using Earth as a reference)
Last edited by phillipsjk on Mon Dec 28, 2009 8:21 am UTC, edited 1 time in total.
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Re: "Gravity Wells" Discussion

Postby skeptical scientist » Mon Dec 28, 2009 8:19 am UTC

hogri wrote:If there is a meaning, what does it mean if I just escaped Mercury's gravitational force (i.e. I'm on the top edge of it's well) and I want to go to Venus? Do I have to consume much more energy to get to the top of the steep edge and the top of Venus's well?

My guess is that objects which are inside* the gravitational wells of other objects are placed in those corresponding wells, at the right height to show how deep they are in each others wells. Every object is inside the gravity well of the sun, and presumably the outermost bump shows the height of the sun's gravity well.

*For a reasonable definition of inside.
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Re: "Gravity Wells" Discussion

Postby TheChewanater » Mon Dec 28, 2009 8:21 am UTC

This comic seems like something you would find on Wikipedia... if (when) Randall controlled the Internet.
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Re: "Gravity Wells" Discussion

Postby The Old Wolf » Mon Dec 28, 2009 8:35 am UTC

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Re: "Gravity Wells" Discussion

Postby Zozoped » Mon Dec 28, 2009 8:36 am UTC

Wonderful physical explanation.

However, what are the odds for all these bodies to be perfectly aligned like that? I mean, come on, Voyager should already have crashed in such a 1D gravity well!

Randall, go 3D!

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Re: "Gravity Wells" Discussion

Postby MDark.Math » Mon Dec 28, 2009 8:41 am UTC

EcoReck20 wrote:
MDark.Math wrote:Wow, now he's just showing off


You seem to have something against these large posters. :(


Nope they're awesome.

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Re: "Gravity Wells" Discussion

Postby Hitaro0 » Mon Dec 28, 2009 8:52 am UTC

Whaa, a great comic that isn't explicitly because of a joke or punchline of sorts? I love it.

chocolate.razorblades wrote:I would love to have Randall as a physics teacher.

I'd love just knowing the guy.

Also a little lol to the Pluto sadness.

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Re: "Gravity Wells" Discussion

Postby philip1201 » Mon Dec 28, 2009 9:13 am UTC

You won't be able to escape Deimos with a bike and a ramp: the bike accelerates because of friction with the surface. You get the friction because gravity pulls you to the ground. WIth as little gravity as on Deimos, every little bump you hit will launch you into the air, so you won't be able to accelerate properly before you reach the ramp. If you do manage to get close to escape velocity, you could be launched into a stable orbit by a pebble-sized bump. The baseball is pretty doable though.

I don't get the "an even more glorious dawn awaits".

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Re: "Gravity Wells" Discussion

Postby Pfhorrest » Mon Dec 28, 2009 9:22 am UTC

philip1201 wrote:I don't get the "an even more glorious dawn awaits".

It really should be "a still more glorious dawn awaits".

See second to last paragraph here (Google cache for highlighting).

Or a video or (most awesome) a music video.
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Re: "Gravity Wells" Discussion

Postby axilog14 » Mon Dec 28, 2009 9:32 am UTC

sje46 wrote:
Ferahgo wrote:
sje46 wrote:The things making noise on Titan...is that a Vonnegut reference?


That was my thought also. If it is, then well done, as that's one of my favorite books ever.

One of mine as well. Amazing science-fiction. This is what I'm talking about, if anyone's wondering: http://en.wikipedia.org/wiki/Sirens_of_titan

The little weeoooeeoooeeeooo was my favoritest part of the comic, and I've never even heard of that book. Now I want to read it! Thanks Randall! :D
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Re: "Gravity Wells" Discussion

Postby PhatPhungus » Mon Dec 28, 2009 10:00 am UTC

bakert wrote:I much preferred the comic to this thread.



Seconded. I love the comics that are just interesting.
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Re: "Gravity Wells" Discussion

Postby oz1cz » Mon Dec 28, 2009 10:09 am UTC

freespace wrote:Earth's gravity well depth has 3 different figures: 5478, 6000, 6379km. The last one should be the correct one.


That's what my calculations say as well. The 6000 is obviously an approximation, but why 5478? Is it a simple error, or is there something more subtle behind it?

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Re: "Gravity Wells" Discussion

Postby Greyplayer » Mon Dec 28, 2009 10:29 am UTC

Suck it, Pluto!
No one ever liked you anyway. (I mean, it's the God of Death).

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Re: "Gravity Wells" Discussion

Postby Lurking Grue » Mon Dec 28, 2009 10:41 am UTC

What happened to the space elevator? Did it fall down?
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Re: "Gravity Wells" Discussion

Postby vermillious » Mon Dec 28, 2009 11:01 am UTC

I love this one, the little carl sitting there reminds me I still need to finsh watching cosmos..

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Re: "Gravity Wells" Discussion

Postby Beacons! » Mon Dec 28, 2009 11:10 am UTC

Aww come on, misquoting Sagan? FOR SHAME
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Re: "Gravity Wells" Discussion

Postby Diadem » Mon Dec 28, 2009 11:23 am UTC

Why is Mercury's gravity well much deeper than Earth's? Seems wrong.
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Re: "Gravity Wells" Discussion

Postby csghone » Mon Dec 28, 2009 11:38 am UTC

Venus value is applied to Earth!! Correct val in the inset,though.

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Re: "Gravity Wells" Discussion

Postby guayec » Mon Dec 28, 2009 11:44 am UTC

Amazing, as always. Thank you.

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Re: "Gravity Wells" Discussion

Postby atimholt » Mon Dec 28, 2009 12:08 pm UTC

The Old Wolf wrote:Picard: There are 9 planets!

black chupaqueso!

Anyways, I hate to be the inevitable guy asking for the explanation to the joke, but whats the 'an even more glorious dawn awaits' thing? Apparently something to do with those Carl Sagan autotune videos. I guess I could just watch them, but... um... its 4 in the morning, and... um... how many are there? More than one? One?
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Re: "Gravity Wells" Discussion

Postby Ubik » Mon Dec 28, 2009 12:29 pm UTC

To me it looks like Mercury has pretty small gravity well of its own, but it resides deeper in the gravity well of Sun. It is the combined effect of the two (mostly Sun's gravity though) which makes it harder to get from Mercury to anywhere else than into Sun.

It would be interesting to see a computer-generated graph showing the same thing (with and without skipping interplanetary distances), just to compare it to Randall's drawing. The well depths are probably pretty correct, but what interests me is the slopes.

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Re: "Gravity Wells" Discussion

Postby Patteroast » Mon Dec 28, 2009 12:40 pm UTC

As a planetary science nerd, I'm definitely more perturbed about splitting up the Galilean moons than not including Pluto. Callisto, Neptune's moon Triton, and Eris are all bigger than Pluto, in addition to everything that's included in the chart (other than the ultra-zooms for Phobos and Deimos). Also, Saturn's rings seem a little odd, but I'm assuming they're just there to illustrate that yes, that is Saturn. Bonus points though for having Neptune correctly deeper than Uranus, because even though Uranus is larger, Neptune is more massive. :)

I'm sad there isn't a depth listed for each well! Hopefully that will be added? In any case, I'm just as happy to see an interesting poster when I visit xkcd as I am when there's a comic. :)

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Re: "Gravity Wells" Discussion

Postby DragonHawk » Mon Dec 28, 2009 1:16 pm UTC

philip1201 wrote:You won't be able to escape Deimos with a bike and a ramp: the bike accelerates because of friction with the surface. You get the friction because gravity pulls you to the ground.

Kerist, it's the airplane-on-a-treadmill all over again.
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Re: "Gravity Wells" Discussion

Postby TizzyFoe » Mon Dec 28, 2009 1:17 pm UTC

How would a black hole look? No bottom?
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Re: "Gravity Wells" Discussion

Postby sdague » Mon Dec 28, 2009 1:24 pm UTC

My only question: Can we get this one as a poster?

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Re: "Gravity Wells" Discussion

Postby tieftonsklave » Mon Dec 28, 2009 1:44 pm UTC

I sincerely miss a URANUS joke... I mean... we all know it's there... I'd guess most of us (even without having seen ET like 10times) get some kind of smile, especially after the "your mom" joke xD
Or is it a sort of scientist's uranus-joke?
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Re: "Gravity Wells" Discussion

Postby jc » Mon Dec 28, 2009 2:16 pm UTC

ananab wrote:Pluto? Where are you? :'(

Pluto and Callisto ran off together. They're both a bit annoyed at how they've been ignored lately. Charon and Ceres are with them.

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Re: "Gravity Wells" Discussion

Postby Armadillo Al » Mon Dec 28, 2009 2:18 pm UTC

philip1201 wrote:If you do manage to get close to escape velocity, you could be launched into a stable orbit by a pebble-sized bump. The baseball is pretty doable though.

This explains why Doom didn't have grenades or anything else throwable.
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Re: "Gravity Wells" Discussion

Postby Freiberg » Mon Dec 28, 2009 2:27 pm UTC

I know that there weren't many jokes here, but I really liked this comic, to be honest.

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Re: "Gravity Wells" Discussion

Postby knowman » Mon Dec 28, 2009 2:55 pm UTC

Man, you people sure nitpick a lot. There is soooo much right with that comic.

Thanks, Randall!!!

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Re: "Gravity Wells" Discussion

Postby jrock91 » Mon Dec 28, 2009 2:56 pm UTC

Ubik wrote:It would be interesting to see a computer-generated graph showing the same thing (with and without skipping interplanetary distances), just to compare it to Randall's drawing. The well depths are probably pretty correct, but what interests me is the slopes.


It's basically impossible to have a diagram that shows planetary sizes and interplanetary distances to the same scale. If you scale the Earth to one cm diameter, the Sun-Earth distance is roughly 10 m, and the Sun-Neptune distance is hundreds of meters.

Or did you mean a diagram that shows interplanetary distances to scale but does NOT show planetary sizes?

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Re: "Gravity Wells" Discussion

Postby neoliminal » Mon Dec 28, 2009 3:28 pm UTC

Your mom can't get off on your anus.

Wait, that's not how puns work. Dammit. :(
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