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Random Science Questions

Blogs > 4iner
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1 2 Next All
airtown
Profile Blog Joined May 2009
United States410 Posts
Last Edited: 2010-11-27 23:47:44
November 27 2010 23:35 GMT
#1
1. If a reinforced hole was drilled straight through the moon and a ladder was added, would the pull of gravity increase or decrease as you climbed down towards the center?

2. What happens if you heat up wood to very high temperatures inside a chamber without oxygen (edit: not necessarily a vacuum, perhaps a chamber with helium or something)?

***
http://www.gwu.edu/~nsarchiv/news/20010430/northwoods.pdf
Adeny
Profile Blog Joined January 2009
Norway1233 Posts
November 27 2010 23:41 GMT
#2
Homework? Pretty interesting questions regardless.

1 - I'd think the pull would stay the same but distributed in other directions as you're leaving some of the mass behind you.
2 - IDK.
ZeaL.
Profile Blog Joined April 2009
United States5955 Posts
November 27 2010 23:42 GMT
#3
1) Gravity should increase proportional to r^2
2) ??
Archas
Profile Blog Joined July 2010
United States6531 Posts
November 27 2010 23:42 GMT
#4
If this is a homework thread, it's pretty subtly prompted. O_o

Question 1 intrigues me greatly; I never thought about it before.

As for question 2, heating wood in a vacuum is very difficult in the first place, since wood is not a good conductor of heat, and the only way to heat wood in a vacuum is through conducting heat radiation through the wood. Assuming you could get it done, there'd be no fire, since fire requires oxygen. The wood would just degrade steadily and release gases; once the process completes, you'd have a very high-grade chunk of charcoal remaining.
The room is ripe with the stench of bitches!
quirinus
Profile Blog Joined May 2007
Croatia2489 Posts
Last Edited: 2010-11-27 23:59:02
November 27 2010 23:42 GMT
#5
1. Decrease I think?

2. It carbonizes? Pyrolyses




Edit:

1. There's a theorem (forgot the name, but some famous physicist showed it) that shows that only the mass under the height you're standing should be used to calculate the pull, so you'd have less mass that pulls you, but you'd be closer to the center. (I think the same theorem says that the mass under your height can be assumed to be located at the center - if it's uniformly spherically distributed) The net result would be that it pulls you less, as obviously, if you were at the center, you'd have to feel no force, as the same force "pulls" you from all directiones.
It's not that hard to show/prove it too.

2. http://en.wikipedia.org/wiki/Charcoal, happened naturally a long time ago...
All candles lit within him, and there was purity. | First auto-promoted BW LP editor.
airtown
Profile Blog Joined May 2009
United States410 Posts
November 27 2010 23:43 GMT
#6
On November 28 2010 08:41 Adeny wrote:
Homework?

Nope.
http://www.gwu.edu/~nsarchiv/news/20010430/northwoods.pdf
hacklebeast
Profile Blog Joined April 2010
United States5090 Posts
Last Edited: 2010-11-27 23:49:48
November 27 2010 23:43 GMT
#7
1, net pull would decreace untill you are weightless in the center
2. it melts, assuming that "very high" means something hotter than humans can make.

the pull decreaces because as you move into the center because all of the matter that you pass starts pulling you backwards. At the center the pull of any matter is perfectly countered by matter on the opposite side, so there is no prevailing foce (ignoring the fact that the moon is not a perfect sphere.)
Protoss: Best, Paralyze, Jangbi, Nal_Ra || Terran: Oov, Boxer, Fantasy, Hiya|| Zerg: Yellow, Zero
MetalFace
Profile Joined September 2010
United States75 Posts
November 27 2010 23:45 GMT
#8
I haven't taken a physics class in years, but from what I remember:

1) I think as you get closer to something, the gravitation attraction you'll feel is stronger, so as you get closer to more of the moon, the pull of gravity would increase.

2) This one I'm not so sure of, but I think it might be called gasification. Pretty much the natural gas in wood can be extracted from it through a process involving heating up wood in a sealed container.

I'm not sure on either of those though. Good luck with whatever project you're doing where you heat up wood in the middle of the moon.
Archas
Profile Blog Joined July 2010
United States6531 Posts
Last Edited: 2010-11-27 23:47:03
November 27 2010 23:46 GMT
#9
On November 28 2010 08:45 MetalFace wrote:
I'm not sure on either of those though. Good luck with whatever project you're doing where you heat up wood in the middle of the moon.

It's clearly a terrorist plot to turn the Moon into a flaming meteor and crash it into Washington D.C.
The room is ripe with the stench of bitches!
bbq ftw
Profile Joined September 2010
United States139 Posts
November 27 2010 23:50 GMT
#10
1. If a reinforced hole was drilled straight through the moon and a ladder was added, would the pull of gravity increase or decrease as you climbed down to the center?

Pretty sure it decreases.

Logic is a bit iffy but:

Gravity at surface of moon is some finite value.
Gravity at the middle of the moon is zero (there's no mass within the sphere defined by r=0)

Assuming that the gravity vs radius is continuous (no abrupt breaks) one can surmise gravity is decreasing.

In addition, bad over simplified math says:

the sphere defined by r=x where x is your current position has volume proportional to x^3. Sphere volume assuming constant density, therefore volume proportional to mass, mass proportional to x^3

the denominator of gravity term is x^2 -- x is also the distance from moon's center of mass (center) from current position

gravity proportional to x^3/x^2 = x

lower x, lower gravity = decreases


2. interesting question, i have no idea.
Slayer91
Profile Joined February 2006
Ireland23335 Posts
Last Edited: 2010-11-27 23:58:56
November 27 2010 23:57 GMT
#11
On November 28 2010 08:45 MetalFace wrote:
I haven't taken a physics class in years, but from what I remember:

1) I think as you get closer to something, the gravitation attraction you'll feel is stronger, so as you get closer to more of the moon, the pull of gravity would increase.


As you reach the dead centre, the gravitational affects of each part of mass will all cancel out, and you'd feel no force.
It is stronger with respect to r^2 only if the mass pulling you is all in one direction.

I assume the wood would vapourise eventually, even without combustion.
AcrossFiveJulys
Profile Blog Joined September 2005
United States3612 Posts
November 27 2010 23:59 GMT
#12
1) gravitational pull will linearly decrease as you approach the center until the pull is zero. Intuitively, this is because the volume "under" you decreases cubically as you move towards the center while the gravitational pull (of the decreasing mass under you) increases quadratically as you move towards the center.
Blisse
Profile Blog Joined July 2010
Canada3710 Posts
Last Edited: 2010-11-28 00:04:24
November 28 2010 00:01 GMT
#13
1. Would need to ask which pull of gravity, Earth's, or the Moon's. Questioning why the same question could not have been Earth instead of the Moon.


FG = (G x m1 x m2) / r^2 have anything to do with this? Cause I don't know if UND means infinite or zero...

2. Do you mean if say you placed it on an electric heater or something similar?
There is no one like you in the universe.
sheaRZerg
Profile Blog Joined June 2009
United States613 Posts
Last Edited: 2010-11-28 00:05:15
November 28 2010 00:03 GMT
#14
On November 28 2010 08:42 quirinus wrote:
There's a theorem (forgot the name, but some famous physicist showed it) that shows that only the mass under the height you're standing should be used to calculate the pull, so you'd have less mass that pulls you, but you'd be closer to the center.


You can use Gauss' law for gravity, much like you would for the electric charge. That is what you are describing for part 1. So the gravitational acceleration would be less on the interior of the sphere than at the surface.

Edit: http://en.wikipedia.org/wiki/Gauss'_law_for_gravity
"Dude, just don't listen to what I say; listen to what I mean." -Sean Plott
airtown
Profile Blog Joined May 2009
United States410 Posts
November 28 2010 00:05 GMT
#15
On November 28 2010 09:01 vica wrote:
1. Would need to ask which pull of gravity, Earth's, or the Moon's. Questioning why the same question could not have been Earth instead of the Moon.

2. Do you mean if say you placed it on an electric heater or something similar?

1. Ignore any outside pulls of gravity

2. Um, idk. Does it matter?
http://www.gwu.edu/~nsarchiv/news/20010430/northwoods.pdf
airtown
Profile Blog Joined May 2009
United States410 Posts
November 28 2010 00:10 GMT
#16
Found this, which seems to be relevant: http://en.wikipedia.org/wiki/Shell_theorem#Inside_a_shell

Can someone put it into laymans terms? I'm only in algebra 2.
http://www.gwu.edu/~nsarchiv/news/20010430/northwoods.pdf
MoltkeWarding
Profile Joined November 2003
5195 Posts
November 28 2010 00:16 GMT
#17
The pull of gravity would increase, but be more evenly distributed in different directions, so there would be a net decrease, assuming that the mass of the moon were equally distributed throughout its body. If we can imagine a moon with its mass heavily concentrated at the core, it's possible that the net pull would increase.
MetalFace
Profile Joined September 2010
United States75 Posts
November 28 2010 00:16 GMT
#18
"net gravitational forces acting on the point mass from the mass elements of the shell cancel out"
-from the wiki page you linked.

Pretty much, the forces of gravity inside a sphere will cancel each other out, like everyone's pretty much been saying. So as you go inside the moon, the forces will be stronger, but the directions will start to cancel each other out, until you get to the middle where all of the forces will be cancelled.
SpiritoftheTunA
Profile Blog Joined August 2006
United States20903 Posts
November 28 2010 00:16 GMT
#19
you don't need the shell theorem, but the general idea of the answer to problem 1 is that any mass distributed outside of the radius you're currently at (say the moon's radius is 10 and you're at 8), you'll feel the pull of only the stuff below you (the radius 8 sphere, not the full moon). this is a linear relationship, so basically

if 'a' is the radius of the moon, and the gravitational pull at the surface is F = G*m1*m2/a^2, you just have to multiply the whole thing by a factor of your radius from the center in order to get the gravitational force for anything under the surface.

so ends up working out as F = r*G*m1*m2*/a^2 (where r is the only variable in this case, your radial distance from the center of the moon)
posting on liquid sites in current year
SpiritoftheTunA
Profile Blog Joined August 2006
United States20903 Posts
Last Edited: 2010-11-28 00:48:24
November 28 2010 00:21 GMT
#20
On November 28 2010 09:16 MoltkeWarding wrote:
The pull of gravity would increase, but be more evenly distributed in different directions, so there would be a net decrease, assuming that the mass of the moon were equally distributed throughout its body. If we can imagine a moon with its mass heavily concentrated at the core, it's possible that the net pull would increase.

This actually isn't the case, even if 90% of the moons' mass were within its innermost 10% sphere, the gravitational pull would decrease as you go towards the center because you still will no longer feel the downward pull of all of the ground you're digging under. as long as the mass is distributed radially symmetrically, your gravitational attraction towards the very center should never increase as you dig into the mass. (actually it might not increase towards the center no matter what)


EDIT: actually that was severely wrong see post below me for details
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