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Author Topic:   Throwing Stuff Down A Mineshaft
Parasomnium
Member
Posts: 2224
Joined: 07-15-2003


Message 31 of 69 (493375)
01-08-2009 12:35 PM
Reply to: Message 23 by Stile
01-08-2009 10:50 AM


Re: Thought experiment
And taking this massive-concept to a hollow sphere... if it were massive enough, it would pull your body apart and splatter you all over the inside walls
I'm not convinced that this would be what happened. If the forces from all sides cancel out, as you explain, then the net force on you would be zero.
The rest of what you say makes excellent sense.

"Ignorance more frequently begets confidence than does knowledge: it is those who know little, not those who know much, who so positively assert that this or that problem will never be solved by science." - Charles Darwin.
Did you know that most of the time your computer is doing nothing? What if you could make it do something really useful? Like helping scientists understand diseases? Your computer could even be instrumental in finding a cure for HIV/AIDS. Wouldn't that be something? If you agree, then join World Community Grid now and download a simple, free tool that lets you and your computer do your share in helping humanity. After all, you are part of it, so why not take part in it?

This message is a reply to:
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cavediver
Member (Idle past 3643 days)
Posts: 4129
From: UK
Joined: 06-16-2005


Message 32 of 69 (493379)
01-08-2009 12:46 PM
Reply to: Message 30 by Parasomnium
01-08-2009 11:42 AM


Re: Thought experiment
But I was under the impression that you can do the maths for the gravitational effects of any massive object by treating it as a point mass.
Only if you are located outside the shell. Once inside the shell, you experience no force whatsoever at any point. There is no "floating" towards the centre.
It's straightforward calculus, just integrating hoops from pole to pole.

This message is a reply to:
 Message 30 by Parasomnium, posted 01-08-2009 11:42 AM Parasomnium has replied

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cavediver
Member (Idle past 3643 days)
Posts: 4129
From: UK
Joined: 06-16-2005


Message 33 of 69 (493380)
01-08-2009 12:52 PM
Reply to: Message 23 by Stile
01-08-2009 10:50 AM


Re: Thought experiment
Inside the hollow part, you have all the tiny points actually pulling you away towards the surface. Given a perfect sphere, each point on one side has an equal point on the exact opposite side that cancels out. Add in a bit of air resistance and no matter where you are in the centre area, you'll eventually "float" to the centre of the sphere.
No, there is no tendancy towards the centre, nor any other point. There is zero gravitational force on the test mass inside the spherical shell.
*Note: By "cancels out" we don't mean that the forces go away. It's that the forces are equal-but-opposite. The forces are all still there, and all as strong as they were before, it's just that there's no movement as a result of those forces.
Hmmm... if you have two sine waves of enormous amplitude coincide out of phase, would you say that there are are equal and opposite waves at those points, or that there are no waves at those points

This message is a reply to:
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Replies to this message:
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cavediver
Member (Idle past 3643 days)
Posts: 4129
From: UK
Joined: 06-16-2005


Message 34 of 69 (493381)
01-08-2009 12:56 PM
Reply to: Message 25 by Stile
01-08-2009 10:58 AM


Re: Thought experiment
I, um.. would like to see the integration on this
I'm going to cheat and refer you here as I hate trying to typeset maths at EvC
'm assuming that you're not "weightless" if you're being pulled towards the centre of mass... you just feel like you have very little weight. But the only place you actually feel truly "weightless" would be at the centre of mass, no?
No, you are truly weightless, and would be in this state anywhere inside the shell.

This message is a reply to:
 Message 25 by Stile, posted 01-08-2009 10:58 AM Stile has replied

Replies to this message:
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Stile
Member
Posts: 4295
From: Ontario, Canada
Joined: 12-02-2004


Message 35 of 69 (493382)
01-08-2009 1:01 PM
Reply to: Message 34 by cavediver
01-08-2009 12:56 PM


Re: Thought experiment
cavediver writes:
I'm going to cheat and refer you here
Ah yes, thanks for the refresher, that's all I needed to see. Of course, there's more mass on "one side", but it's farther away. And the cool properties of a perfect sphere balance this distance-mass relationship perfectly equally.
I understand. Yes, even if you're at a spot 90m from the centre (back to my scenario), you wouldn't move due to any gravitational forces, and therefore be weightless.

This message is a reply to:
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Stile
Member
Posts: 4295
From: Ontario, Canada
Joined: 12-02-2004


Message 36 of 69 (493383)
01-08-2009 1:05 PM
Reply to: Message 33 by cavediver
01-08-2009 12:52 PM


Re: Thought experiment
cavediver writes:
Hmmm... if you have two sine waves of enormous amplitude coincide out of phase, would you say that there are are equal and opposite waves at those points, or that there are no waves at those points
Perhaps I am simplifying gravity too much. I take it the gravitational forces act via waves, then?
I was thinking more like if I have a force of 10N pulling my left arm and 10N pulling my right arm, I don't move (they "cancel out"), but both forces are still there.
Am I simplifying gravity too much then, by reducing it's force to a mere number of "x Newtons"?

This message is a reply to:
 Message 33 by cavediver, posted 01-08-2009 12:52 PM cavediver has replied

Replies to this message:
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Larni
Member (Idle past 164 days)
Posts: 4000
From: Liverpool
Joined: 09-16-2005


Message 37 of 69 (493384)
01-08-2009 1:10 PM
Reply to: Message 24 by Stile
01-08-2009 10:53 AM


Re: Thought experiment
Thanks for clearing that one up, Stile.
I really must stop thinking, now: my head hurts

This message is a reply to:
 Message 24 by Stile, posted 01-08-2009 10:53 AM Stile has replied

Replies to this message:
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Larni
Member (Idle past 164 days)
Posts: 4000
From: Liverpool
Joined: 09-16-2005


Message 38 of 69 (493385)
01-08-2009 1:15 PM


How wonderful
How wonderful that we can get from goats to Dyson spheres in less than 40 post.
EvC is a really odd place

  
Blue Jay
Member (Idle past 2698 days)
Posts: 2843
From: You couldn't pronounce it with your mouthparts
Joined: 02-04-2008


Message 39 of 69 (493386)
01-08-2009 1:19 PM


Scientists in their spare time
Just to let all the creationists at EvC know, this is what scientists do in their spare time.

I'm Bluejay.
Darwin loves you.

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Stile
Member
Posts: 4295
From: Ontario, Canada
Joined: 12-02-2004


Message 40 of 69 (493387)
01-08-2009 1:19 PM
Reply to: Message 37 by Larni
01-08-2009 1:10 PM


Re: Thought experiment
No problem, but make sure to read my Message 35 in this thread, I'm pretty sure I'm screwing up the "inside the hollow sphere" stuff as cavediver is explaining.
I got carried away and forgot the relationship with mass vs distance.
I was thinking of gravity as too much of a "linear force" when it's more of a linear force that has a fundamental basis as a wave. In which case, what I said about the forces cancelling out but still being there is totally incorrect, and the forces would actually cancel out so that there is no "feeling" of them at all.
This is not true for energy like Kinetic energy (someone actually pulling on my arm)... but I think Potential energy works a bit different and actually does cancel out down to absolute 0 force as opposed to simply 0 net force.
See cavediver's reply to my question and I'm pretty sure he's about to confirm that forces due to gravity act as waves.
I should be able to confirm that (I found Newtonian physics extremely intuitive in school), but apparently it's been a bit too long since college for me
..which makes me a bit sad, it's only been 9-10 years

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Stile
Member
Posts: 4295
From: Ontario, Canada
Joined: 12-02-2004


Message 41 of 69 (493388)
01-08-2009 1:23 PM
Reply to: Message 39 by Blue Jay
01-08-2009 1:19 PM


Re: Scientists in their spare time
Mantis writes:
Just to let all the creationists at EvC know, this is what scientists do in their spare time.
Next there'll be a conversation on how much the water level of the ocean changes (if at all) when you throw an anchor overboard...

This message is a reply to:
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cavediver
Member (Idle past 3643 days)
Posts: 4129
From: UK
Joined: 06-16-2005


Message 42 of 69 (493390)
01-08-2009 1:35 PM
Reply to: Message 36 by Stile
01-08-2009 1:05 PM


Re: Thought experiment
I take it the gravitational forces act via waves, then?
No more or less than any other force. Gravity is great because it is the real deal - a fundemental force that we can see up close. E/M we can too, if we have a pair of magnets. But E/M is behind just about every other force we encounter, whether it is lifting weights, pushing doors, trains pulling trucks, etc. The large scale mechanical forces are simply infinitely tangled webs of electromagnetic interactions of which we have no awareness. And fundemental forces simply come down to numbers. What is the force's value at this point in space. At that point, there is no way to tell whether a value of +15 is made of +25 and -10, +1000000000000015 and -1000000000000000, etc.

This message is a reply to:
 Message 36 by Stile, posted 01-08-2009 1:05 PM Stile has replied

Replies to this message:
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Parasomnium
Member
Posts: 2224
Joined: 07-15-2003


Message 43 of 69 (493392)
01-08-2009 1:52 PM
Reply to: Message 32 by cavediver
01-08-2009 12:46 PM


Re: Thought experiment
cavediver writes:
Parasomnium writes:
But I was under the impression that you can do the maths for the gravitational effects of any massive object by treating it as a point mass.
Only if you are located outside the shell. Once inside the shell, you experience no force whatsoever at any point. There is no "floating" towards the centre.
It's straightforward calculus, just integrating hoops from pole to pole.
Thanks for enlightening me, Cavediver, I didn't know that. If I'm correct you're a physicist of some kind (I'm too lazy to look it up in old messages), so I'll take your word for it.

"Ignorance more frequently begets confidence than does knowledge: it is those who know little, not those who know much, who so positively assert that this or that problem will never be solved by science." - Charles Darwin.
Did you know that most of the time your computer is doing nothing? What if you could make it do something really useful? Like helping scientists understand diseases? Your computer could even be instrumental in finding a cure for HIV/AIDS. Wouldn't that be something? If you agree, then join World Community Grid now and download a simple, free tool that lets you and your computer do your share in helping humanity. After all, you are part of it, so why not take part in it?

This message is a reply to:
 Message 32 by cavediver, posted 01-08-2009 12:46 PM cavediver has not replied

  
Stile
Member
Posts: 4295
From: Ontario, Canada
Joined: 12-02-2004


Message 44 of 69 (493394)
01-08-2009 1:54 PM
Reply to: Message 42 by cavediver
01-08-2009 1:35 PM


Where's the difference?
I think I understand... let me ramble a bit and if you could confirm what I'm saying it would be greatly appreciated.
All forces act in the same manner.
Forces seemingly cancel each other out as waves would do when they act on the same "spot".
Forces due to Gravity (Potential energy) act "everywhere" (in their mass vs distance relationship).
Forces due to Kinetic energy act locally on the spot they interact.
I think that was my confusion. The force of gravity being "everywhere" vs. local forces due to something's mass running into something else's mass.
All forces act the same, but if I have a planet's gravity pulling on my left and an equal planet's gravity pulling on my right... I don't get ripped apart between the two because the "everywhere" force due to their gravities actually cancels out.
All forces act the same, but if I have a truck pulling on my left and an equal truck pulling on my right... I get ripped apart because this Kinetic energy is focused locally on my arms, which itself comes down to fields interacting. However, the interactions of these fields are more local and not so pervasively "everywhere" such as the interaction due to a gravitational force.
Something like that?

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DevilsAdvocate
Member (Idle past 3101 days)
Posts: 1548
Joined: 06-05-2008


Message 45 of 69 (493406)
01-08-2009 4:44 PM
Reply to: Message 42 by cavediver
01-08-2009 1:35 PM


Re: Thought experiment
No more or less than any other force. Gravity is great because it is the real deal - a fundemental force that we can see up close. E/M we can too, if we have a pair of magnets. But E/M is behind just about every other force we encounter, whether it is lifting weights, pushing doors, trains pulling trucks, etc. The large scale mechanical forces are simply infinitely tangled webs of electromagnetic interactions of which we have no awareness.
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, gravitational forces from matter can effectively nullify the gravity forces from other matter resulting in effectively a near 0 g effect but this is not the same as an attractive and a repulsive force i.e. electromagnetism cancelling each other out on the microscopic realm resulting in a very short range of reaction with matter. Hope this makes sense.

For me, it is far better to grasp the Universe as it really is than to persist in delusion, however satisfying and reassuring.
Dr. Carl Sagan

This message is a reply to:
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Replies to this message:
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