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Author Topic:   Throwing Stuff Down A Mineshaft
Rrhain
Member (Idle past 378 days)
Posts: 6349
From: San Diego, CA, USA
Joined: 05-03-2003


Message 61 of 69 (493637)
01-09-2009 11:32 PM
Reply to: Message 53 by cavediver
01-09-2009 7:50 AM


cavediver responds to me:

quote:
Relative to the shell, it doesn't matter what the external gravitational field is. The shell will respond to that field identically to the shell occupant, and again the occupant will be entirely weightless within the shell.

But only with respect to the shell. I was trying to give an image of how the person could be floating within the shell.

That is, assuming an empty universe except for the moon, you would be gravitationally pulled toward the surface of the moon if you were outside it.

Inside it, however, you wouldn't be pulled toward any surface: You could float around inside it.

If, however, you had both the moon as a spherical shell and there was the earth around it, you would feel the gravitational pull of the earth. If the moon is in freefall, then so are you within it and thus you don't feel anything.

But if the moon were sitting on the earth, you would be standing on the shell were it was touching and would not be able to float to the top.

That's why you don't float to the top of the house when you enter your home.


Rrhain

Thank you for your submission to Science. Your paper was reviewed by a jury of seventh graders so that they could look for balance and to allow them to make up their own minds. We are sorry to say that they found your paper "bogus," specifically describing the section on the laboratory work "boring." We regret that we will be unable to publish your work at this time.

This message is a reply to:
 Message 53 by cavediver, posted 01-09-2009 7:50 AM cavediver has responded

Replies to this message:
 Message 62 by cavediver, posted 01-10-2009 6:09 AM Rrhain has responded

  
cavediver
Member (Idle past 2149 days)
Posts: 4129
From: UK
Joined: 06-16-2005


Message 62 of 69 (493658)
01-10-2009 6:09 AM
Reply to: Message 61 by Rrhain
01-09-2009 11:32 PM


But if the moon were sitting on the earth, you would be standing on the shell were it was touching and would not be able to float to the top.

Obviously, in this case you are no longer simply inside a spherical shell, but a very lopsided mass distribution :)

Not quite what you said, with

Rrhain writes:

Indeed, assuming that there is no other gravitational field anywhere else in the universe


This message is a reply to:
 Message 61 by Rrhain, posted 01-09-2009 11:32 PM Rrhain has responded

Replies to this message:
 Message 63 by Rrhain, posted 01-10-2009 9:48 PM cavediver has responded

  
Rrhain
Member (Idle past 378 days)
Posts: 6349
From: San Diego, CA, USA
Joined: 05-03-2003


Message 63 of 69 (493804)
01-10-2009 9:48 PM
Reply to: Message 62 by cavediver
01-10-2009 6:09 AM


cavediver responds to me:

quote:
Obviously, in this case you are no longer simply inside a spherical shell, but a very lopsided mass distribution

Not quite what you said, with

Rrhain writes:

Indeed, assuming that there is no other gravitational field anywhere else in the universe


And if there were a gravitational field without the shell actually touching anything else, what then? No, not in free fall...the shell isn't moving, but there is a gravitational field in a certain direction external to the shell. That field would affect anything inside the shell.

And people wonder why getting published is so hard. You have to submit your work to others to have them point out things you've overlooked (thank you, cavediver.) ;)

Edited by Rrhain, : No reason given.


Rrhain

Thank you for your submission to Science. Your paper was reviewed by a jury of seventh graders so that they could look for balance and to allow them to make up their own minds. We are sorry to say that they found your paper "bogus," specifically describing the section on the laboratory work "boring." We regret that we will be unable to publish your work at this time.

This message is a reply to:
 Message 62 by cavediver, posted 01-10-2009 6:09 AM cavediver has responded

Replies to this message:
 Message 64 by cavediver, posted 01-11-2009 5:14 AM Rrhain has responded

  
cavediver
Member (Idle past 2149 days)
Posts: 4129
From: UK
Joined: 06-16-2005


Message 64 of 69 (493845)
01-11-2009 5:14 AM
Reply to: Message 63 by Rrhain
01-10-2009 9:48 PM


And if there were a gravitational field without the shell actually touching anything else, what then? No, not in free fall...the shell isn't moving...

Er, what do you mean by not "moving"? How is it not in free-fall? Does it now have engines strapped to the shell which are firing? ;)

Edited by cavediver, : No reason given.


This message is a reply to:
 Message 63 by Rrhain, posted 01-10-2009 9:48 PM Rrhain has responded

Replies to this message:
 Message 65 by Rrhain, posted 01-11-2009 5:31 PM cavediver has responded

  
Rrhain
Member (Idle past 378 days)
Posts: 6349
From: San Diego, CA, USA
Joined: 05-03-2003


Message 65 of 69 (493916)
01-11-2009 5:31 PM
Reply to: Message 64 by cavediver
01-11-2009 5:14 AM


cavediver responds to me:

quote:
Er, what do you mean by not "moving"? How is it not in free-fall? Does it now have engines strapped to the shell which are firing?

I can think of a few ways to have the shell not moving. Perhaps the shell is magnetic and there is a magnetic force sufficient to counteract the gravitational pull. If there is something not affected by the magnetic field inside the shell, it would be standing on the edge of the shell closest to the gravitational field.

Similarly, there could be some sort of fluid surrounding the shell providing buoyancy.

The point I was trying to make is that under the influence of just the shell, a spherical shell's geometry provides perfect gravitational counterbalance in all directions internally. Perhaps I should have phrased it that way.


Rrhain

Thank you for your submission to Science. Your paper was reviewed by a jury of seventh graders so that they could look for balance and to allow them to make up their own minds. We are sorry to say that they found your paper "bogus," specifically describing the section on the laboratory work "boring." We regret that we will be unable to publish your work at this time.

This message is a reply to:
 Message 64 by cavediver, posted 01-11-2009 5:14 AM cavediver has responded

Replies to this message:
 Message 66 by cavediver, posted 01-11-2009 6:10 PM Rrhain has not yet responded

  
cavediver
Member (Idle past 2149 days)
Posts: 4129
From: UK
Joined: 06-16-2005


Message 66 of 69 (493920)
01-11-2009 6:10 PM
Reply to: Message 65 by Rrhain
01-11-2009 5:31 PM


Perhaps the shell is magnetic

Yes, I even started to mention this possibility in my last reply but couldn't be arsed :)

The point I was trying to make is that under the influence of just the shell, a spherical shell's geometry provides perfect gravitational counterbalance in all directions internally.

Yes, your initial wording:

Indeed, assuming that there is no other gravitational field anywhere else in the universe, there is no gravitation pull inside a perfectly uniform, spherical shell.

explicity denies the essential "Faraday Cage"-like nature of the shell. It actually shields you from knowledge of the external gravitational fields, by nature of the shell acting as a point source to external fields, i.e. it responds the same as you do, inside the shell. This is what I am trying to emphasise. The fact that you can introduce other fields/fluids or change the mass distribution is irrelevant.


This message is a reply to:
 Message 65 by Rrhain, posted 01-11-2009 5:31 PM Rrhain has not yet responded

  
cavediver
Member (Idle past 2149 days)
Posts: 4129
From: UK
Joined: 06-16-2005


Message 67 of 69 (493924)
01-11-2009 6:46 PM
Reply to: Message 45 by DevilsAdvocate
01-08-2009 4:44 PM


Re: Thought experiment
Sorry, missed this at the time.

Correct me if I am wrong but isn't the reason that gravity acts at a more macro scale than the other fundemental forces i.e. weak, strong and electromagnetic is that gravity acts in one direction i.e. attraction and does not cancel itself out?

Yes, that is correct. Even though gravity is exceptionally weak compared to the other forces, it cannot be shielded. An electromagnetic charge, by its nature, attracts opposite charges that will effectively shield the original charge - atoms being the perfect example of this. Mass attracts more mass, which simply builds the attraction. This actually lies behind the reason for our existence - the reason why compact distributions have such low entropy, contrary to naive considerations of thermodynamics. But this is a big topic not for this thread!


This message is a reply to:
 Message 45 by DevilsAdvocate, posted 01-08-2009 4:44 PM DevilsAdvocate has not yet responded

  
shalamabobbi
Member (Idle past 1355 days)
Posts: 397
Joined: 01-10-2009


Message 68 of 69 (494917)
01-20-2009 2:46 AM
Reply to: Message 34 by cavediver
01-08-2009 12:56 PM


Re: Thought experiment
I think the easier way to picture the integration inside the shell is the following:

Think of a slender cone emanating from you in opposite directions. Where this cone intercepts the sphere surrounding you is the mass pulling you in that direction. But the amount of mass intercepted and the gravitational pull of that mass vary as distance squared. Gravity is diminished by distance squared. The mass intercepted is increased by distance squared. So the effect of the pull from the window of this cone is constant and exactly countered by the effect of the intercepted mass of the cone directed oppositely to the first. In the limit as the steradian measure of this cone is diminished the results become exactly accurate. This nullifies the effects of the cone not being perpendicular to the shell at all summation orientations.


This message is a reply to:
 Message 34 by cavediver, posted 01-08-2009 12:56 PM cavediver has not yet responded

Replies to this message:
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shalamabobbi
Member (Idle past 1355 days)
Posts: 397
Joined: 01-10-2009


Message 69 of 69 (494918)
01-20-2009 3:08 AM
Reply to: Message 68 by shalamabobbi
01-20-2009 2:46 AM


Re: Thought experiment
Actually the angles of interception I think are the same, as any straight line is a chord in a circle and a great circle can be drawn on the sphere through any two point of interception. Sorry, a little rusty.

This message is a reply to:
 Message 68 by shalamabobbi, posted 01-20-2009 2:46 AM shalamabobbi has not yet responded

  
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