What's on the other side?

That is really cool. I'm directly opposite Gibralter (give or take a few miles).

Chris
 
cwyckham said:
That is really cool. I'm directly opposite Gibralter (give or take a few miles).

Chris
Click to expand...

Would you ever have guess that?

I grew up with folks telling me if I dug in the backyard too deep, I'd wind up in China :P I'm now finding out, I'd probably have drown.

Best,

Dave
 
Dave Siciliano said:
Would you ever have guess that?

I grew up with folks telling me if I dug in the backyard too deep, I'd wind up in China :P I'm now finding out, I'd probably have drown.

Best,

Dave
Click to expand...

Yeah, I'd have ended up in the Indian Ocean for my efforts. Turns out you have to be in Argentina to get to China!

Chris
 
yea when we were kids we didnt understand that whole sphere thing. with the US and China both in the northern hemisphere, it would be kinda tricky...
 
So, if I turn the RG on end and start burning a hole between the runways, I'll end up underwater somewhere west of Oz. Heck, I can do that by dropping power just a couple miles west of me, here.
 
tonycondon said:
yea when we were kids we didnt understand that whole sphere thing. with the US and China both in the northern hemisphere, it would be kinda tricky...
Click to expand...

Actually, nothing said you had to pass through the center of the earth. Heck with my sense of direction underground I'd probably end up in Texas.
 
So if you fell into a hole that went to the other side of the earth, would you get stuck in the middle because of gravity? That would suck if the hole was so big that you couldn't reach one of the sides while you were floating in the middle of the planet.
 
nah you'd be char broiled long before you made it to the center of the earth ben :D
 
Wet. Very wet. 100s of miles SE of South Africa. Heck, I can go a couple hundred yards east of here and get just as wet.
 
bkreager said:
So if you fell into a hole that went to the other side of the earth, would you get stuck in the middle because of gravity? That would suck if the hole was so big that you couldn't reach one of the sides while you were floating in the middle of the planet.
Click to expand...

Yes, you'd jump in the hole and fall down towards the center. At the center, you'd be pulled equally in all directions, so you'd feel weightless, but you'd be moving so fast that you'd whiz by. As you went past the center, you'd start to slow down and you'd end up oscillating back and forth around the center. Eventually, the atmospheric drag would slow you down so you'd float in the center.
 
cwyckham said:
Yes, you'd jump in the hole and fall down towards the center. At the center, you'd be pulled equally in all directions, so you'd feel weightless, but you'd be moving so fast that you'd whiz by. As you went past the center, you'd start to slow down and you'd end up oscillating back and forth around the center. Eventually, the atmospheric drag would slow you down so you'd float in the center.
Click to expand...

Ok Mr AeroSpace Engineer dude, I build a Ceramic over Titanium well insulated aerodynamic capsule, say 100kg, to take the trip in. How big of a JATO pod would I need to burn on the entry to be able to escape the other side?
 
cwyckham said:
Yes, you'd jump in the hole and fall down towards the center. At the center, you'd be pulled equally in all directions, so you'd feel weightless, but you'd be moving so fast that you'd whiz by. As you went past the center, you'd start to slow down and you'd end up oscillating back and forth around the center. Eventually, the atmospheric drag would slow you down so you'd float in the center.
Click to expand...
Hmm, interesting. As you fell toward the center, wouldn't your rate of acceleration slow as you began to have as much mass above you as below you?
 
Henning said:
Ok Mr AeroSpace Engineer dude, I build a Ceramic over Titanium well insulated aerodynamic capsule, say 100kg, to take the trip in. How big of a JATO pod would I need to burn on the entry to be able to escape the other side?
Click to expand...

Just enough to compensate for the atmospheric drag assuming you started at the same distance from center at your departure end as it would take to clear the ground on the destination end. Of course to stay on the far side you'd need a way to prevent the return trip from starting once you arrived.
 
Frank Browne said:
Hmm, interesting. As you fell toward the center, wouldn't your rate of acceleration slow as you began to have as much mass above you as below you?
Click to expand...

The acceleration would decrease to zero at the center, but that would be the point of maximum velocity. Think of a pendulum swinging, there's no acceleration at the very bottom of the swing but the velocity maxes out there.
 
Frank Browne said:
Hmm, interesting. As you fell toward the center, wouldn't your rate of acceleration slow as you began to have as much mass above you as below you?
Click to expand...

Yes. At the surface you're at 1g acceleration when the trap door opens. As you descend, the mass above you pulls up on you causing you to decelerate. Damped harmonic oscillator.


As a kid, I never could figure out the China thing that everyone said. I always had maps and a globe around and based on that, knew I would end up in the Indian Ocean.
 
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fgcason said:
Yes. At the surface you're at 1g acceleration when the trap door opens. As you descend, the mass above you pulls up on you causing you to decelerate.
Click to expand...

Close but not quite. You will continue to accelerate towards the center but the rate of acceleration will decrease and reach zero at the center.
 
Ironic that one day in 1979 I left NAS Miramar near San Diego, and ended up in the Indian Ocean one day in 1980... the maps above show the spot opposite San Diego being not too far from where we ended up ...
 
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What kind of childhood did y'all have<g>.

Poor Lance and Tony! All those Bugs Bunny and Road Runner flicks where someone blew them down to China and you didn't believe it?

I'm so sorry y'all grew up so early :rofl:

Best,

Dave
 
Looks like Hawaii and Alaska are the only states where you could hit land on the other side.
 
Henning said:
Ok Mr AeroSpace Engineer dude, I build a Ceramic over Titanium well insulated aerodynamic capsule, say 100kg, to take the trip in. How big of a JATO pod would I need to burn on the entry to be able to escape the other side?
Click to expand...

I accept your challenge, sir!

The atmospheric pressure will be immense towards the center of the Earth. Even though the net gravitational attraction goes to zero at the center, you have the weight of a whole lot of atmosphere above you. It is also very hot, so I'd guess that the air will actually be in a very dense supercritical state where it is neither liquid nor gas. The result will be that your aerodynamic drag will be massive around the same time that your gravitational acceleration goes to zero.

So, even with a really big JATO pod, you're pretty much going to hit terminal velocity before you hit the center of the Earth. The JATO bottle will only effect the details of where you hit terminal velocity. Because the drag is so large, you won't overshoot the center by very much before you get pulled back down.

Therefore, you were obviously asking me a trick question. The answer is to wait until the instant you start to fall back towards the center of the Earth (your apogee), then let off the JATO bottle. Because of the massive drag down there, your most efficient strategy will be to move fairly slowly at first, letting off multiple bottles in series instead of just one bottle.

Did I pass?

Chris
 
AuntPeggy said:
Looks like Hawaii and Alaska are the only states where you could hit land on the other side.
Click to expand...

Just barely for Alaska -the antipode for Barrow, Alaska on the Arctic coast is barely onshore in Antarctica, due south of the African continent. For me here in central Alaska the antipode is still very wet.
 
Wow! If only we could seal the hole in the core; put a little snow on one side! We'd have one heck of a skiing vertical drop! One could reach terminal velocity on the way down; go through the center--do a 180 and ski back!

If only we didn't have to worry about the heat and extra air pressure!!

I guess you are saying, air pressure would be higher in the core because of the weight of the air above it pressing down--right? But I don't get how it could be immense; air only weighs so much; and I don't see how the weight above it would compress it to the level you mention.

That could ruin the skiing <vbg>

Best,

Dave
 
cwyckham said:
I accept your challenge, sir!

The atmospheric pressure will be immense towards the center of the Earth. Even though the net gravitational attraction goes to zero at the center, you have the weight of a whole lot of atmosphere above you. It is also very hot, so I'd guess that the air will actually be in a very dense supercritical state where it is neither liquid nor gas. The result will be that your aerodynamic drag will be massive around the same time that your gravitational acceleration goes to zero.

So, even with a really big JATO pod, you're pretty much going to hit terminal velocity before you hit the center of the Earth. The JATO bottle will only effect the details of where you hit terminal velocity. Because the drag is so large, you won't overshoot the center by very much before you get pulled back down.

Therefore, you were obviously asking me a trick question. The answer is to wait until the instant you start to fall back towards the center of the Earth (your apogee), then let off the JATO bottle. Because of the massive drag down there, your most efficient strategy will be to move fairly slowly at first, letting off multiple bottles in series instead of just one bottle.

Did I pass?

Chris
Click to expand...

I like your answer!:D

Lance has also got the right idea.

The earth above you will not pull you up; all "earth" at a radius greater than yours can be thought of as a "hollow planet" that you are inside. You feel the pull of that part of the earth which is "below you" only. This mass decreases as R^3 (by volume) while the strength of the gravitational force varies according to 1/R^2... as a result, F_g drops linearly from (your weight)----> (zero) as you decend.

Extra credit goes to the first person to prove mathematically that an object inside a hollow planet feels no net gravitational force. The hollow planet can be ANY shape.

--Kath
Resident physics professor

Edit: the earth above you does exert a force... but from all directions added up it contributes no NET force.
 
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Dave Siciliano said:
What kind of childhood did y'all have<g>.

Poor Lance and Tony! All those Bugs Bunny and Road Runner flicks where someone blew them down to China and you didn't believe it?

I'm so sorry y'all grew up so early :rofl:

Best,

Dave
Click to expand...

No i believed it then.
 
Hmm. Disregarding the heat, it seems that you would hit the center, and not be able to escape. Gravity would keep you in the dead center, with no chance to move in any direction, am I wrong?
 
SkyHog said:
Hmm. Disregarding the heat, it seems that you would hit the center, and not be able to escape. Gravity would keep you in the dead center, with no chance to move in any direction, am I wrong?
Click to expand...

You're weightless at the center. All you need is some force to get you over to the side of the hole. So if the hole is small enough, you can just take your shoe off and chuck it in the opposite direction you want to go. You'll drift in the opposite direction. Every inch you drift, you'll gain a small amount of weight in the opposite direction. If you chucked your shoe hard enough, you'll reach the edge of the hole and you can climb up the hole and out.

Of course, it's a long way, and as you get more and more tired out from the climbing, you'll also be getting heavier and heavier.

Chris
 
cwyckham said:
You're weightless at the center. All you need is some force to get you over to the side of the hole. So if the hole is small enough, you can just take your shoe off and chuck it in the opposite direction you want to go. You'll drift in the opposite direction. Every inch you drift, you'll gain a small amount of weight in the opposite direction. If you chucked your shoe hard enough, you'll reach the edge of the hole and you can climb up the hole and out.
Chris
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Or if you had a really big shoe.

Im pretty sure I could do the proof of the hollow planet if I could find my stinkin Astrodynamics Book. I know ive got it around here somewhere but god knows where.

Has anyone else noticed the sharp rise in Physics problems lately on the board? Is this some sort of test or something?
 
kath said:
Extra credit goes to the first person to prove mathematically that an object inside a hollow planet feels no net gravitational force. The hollow planet can be ANY shape.
Click to expand...

I assume that there are other assumptions such as constant thickness and density gradient through the shell? Also, the shape of the shell would have to be such that there's never any material between me and the centroid, right (three dimensional function?)?

Hmmm.... This is very counter-intuitive. So say I'm inside a rectangular planet that is so narrow in one dimension compared to the others that I'm essentially between two parallel, infinite plates. I would have thought that I'd be (net) attracted to whichever plate I'm closest to.

Chris
 
tonycondon said:
Has anyone else noticed the sharp rise in Physics problems lately on the board? Is this some sort of test or something?
Click to expand...

I think we just hit some sort of critical geek mass and things got out of hand. I'm all for it, personally.

Chris
 
kath said:
The earth above you will not pull you up; all "earth" at a radius greater than yours can be thought of as a "hollow planet" that you are inside. You feel the pull of that part of the earth which is "below you" only. This mass decreases as R^3 (by volume) while the strength of the gravitational force varies according to 1/R^2... as a result, F_g drops linearly from (your weight)----> (zero) as you decend.

Extra credit goes to the first person to prove mathematically that an object inside a hollow planet feels no net gravitational force. The hollow planet can be ANY shape.
Click to expand...

Huh? You are correct that it doesn't matter what shape the planet is - but it doesn't make any difference if the planet is solid or hollow, either. Inside a hollow planet the object will still feel a net gravitational force pulling it to the planet's, ahem, center of gravity. The CG, being the point around which the mass is equally distributed, is also the point around which gravitational attraction is equally distributed. The sum of the accelleration vectors from all directions will be zero at the CG, but non-zero anywhere else.

Regards,
Joe
 
Dave Siciliano said:
I guess you are saying, air pressure would be higher in the core because of the weight of the air above it pressing down--right? But I don't get how it could be immense; air only weighs so much; and I don't see how the weight above it would compress it to the level you mention.
Click to expand...

Yeah, you've got the idea right. I'm just guessing that the pressure and heat would be enough for it to go supercritical based on the large numbers involved. As you go down the hole, the pressure in each slice of air will be equal to the pressure above it, plus a bit due to the weight of the new slice. The force of gravity's going down as you descend, but the density of the air is increasing due to the pressure, so you're still adding significant pressure as you descend.

Keep in mind that the atmosphere is generally taken to be about 100 km or 60 miles thick. The Earth's radius is 4000 miles! That's a lot of air to stack up.

Chris
 
Keep in mind that the atmosphere is generally taken to be about 100 km or 60 miles thick. The Earth's radius is 4000 miles! That's a lot of air to stack up.
Chris[/quote]

Thanks! Is the gravitational force getting stronger as one descends toward the center? At what point would it zero out? At that point the gravitational force of the air pressure would be less, wouldn't it? Weight from atmosphere above, but less gravitational force?

Best,

Dave
 
Dave Siciliano said:
Thanks! Is the gravitational force getting stronger as one descends toward the center? At what point would it zero out? At that point the gravitational force of the air pressure would be less, wouldn't it? Weight from atmosphere above, but less gravitational force?

Best,

Dave
Click to expand...

The (net) gravitational force decreases as you head towards the center and becomes zero at the very center. This means that the incremental change in the air pressure as you descend would decrease to zero at the centre. So the air pressure at the center and a foot above the center would be virtually identical. However, you still have the weight of thousands of miles of air pressing down on you, so the highest pressure would be right at the center.

Chris
 
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