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Earth's Gravity

Posted on 2004-03-21
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Say i have this Super heat resistant metal pipe, now i bore a hole in the ground with it that goes all the way to the other end of the earth (right through the center of the earth). now i have the two ends of the pipe sticking one feet above the ground on either side of the earth.
Now two apples are dropped into this pipe, one from either end. so whats gonna happen to these apples once they reach the center, will they just float around in mid air???

R
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Question by:R_Rajesh
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romano70 earned 125 total points
ID: 10646338
Yes.

Assume there is some air resistance. Because the gravity become gradually to zero at the center,
the apples probably gradually slow down before reaching the center. I think the inertia isn't enogh to permit to crush one to the other. Consider that the pressure of the air is very high at the center and this means that there is a high air resistance. I think also, they reach the center very slowly and it's very improbable that they arrive to touch each other.

Assume there is no air resistence. The acceleration go down, but it's always positive before the apple reach the center. So, the velocity became higher and higher. The apples crush or if they missed each other, they go to the other end of the pipe, and they stops exactly at the opposite level they are dropped.
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by:HippyWarlock
HippyWarlock earned 70 total points
ID: 10646547
I think I see your problem:

Gravity is a phenomena that occurs with increasing effect in proportion to mass. ie the moon has a gravity, but less than that of the earth.

As each particle of matter exhibits a gravitational force - and a planet is merely a collection of particles, when your apples near the centre, they are being pulled back up by the mass of the particles above them, as well as down by all the particles below.

As romano700 says, the gravity at some point will be zero. And his/her comments on wind resistance must also be taken into consideration.

Maybe if you had exampled with spheroids some egghead could've done the maths on this :-)

I wonder if you drilled a hole in a black hole, what the apple would do under the extreme tugging of the gravities


HTH - Peace
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by:grg99
grg99 earned 20 total points
ID: 10646661
You may have a problem if your pipe goes thru the center of gravity.  There may be all sorts of strange things that the Earth has captured over the years, maybe some magnetic monopoles, tiny black holes, all kinds of space rubbish.  
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by:ozo
ozo earned 95 total points
ID: 10646794
If the pipe is full of air, the density will probably be enough for the apple to float a few hundred kilometers down
If the pipe is full of vacuum, the apple will oscilate with a period of about 90 minutes
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by:Thibault St john Cholmondeley-ffeatherstonehaugh the 2nd
Thibault St john Cholmondeley-ffeatherstonehaugh the 2nd earned 40 total points
ID: 10648408
The apples will fall for some distance, gradually moving towards one side of the tube, continue for a while sliding and end up by rolling slowly to the half way point where they will remain resting against one side of the tube. Gravity works sideways as well as up and down.
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by:HippyWarlock
ID: 10648539
Just for the sake of argument....

RobinD

The 'Up/Down' is only for reference to either end of the tube, likewise sideways requires an up/down to reference itself.

Why will they rest against one side of the tube? Surely if the tube is central then all sides will exert an equal pull?

As the Earth is denser at its core I would disagree with the halfway resting place.

I'll leave the 'rolling' concept alone ....for now :-)

Peace
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ID: 10648607
HippyWarlock,

I was using up/down as you stated and sideways to be in a plane at right angles to this.
Even the apples are not an 'equal' shape, and the world is only nearly round, there are mountains and seas and other sticky-out bits with different densities.
Even with all things being equal it is probably impossible to drop something straight through the hole.
Try passing a round magnet through a section of slightly larger steel tube, or re-inserting the armature of a starter motor between the magnets - it always pulls to one side. You only need to be a fraction off for the attraction to one side to be greater than the other.
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by:HippyWarlock
ID: 10648663
Assuming a symmetrical Earth along the axis of the tube
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ID: 10648886
Still don't think you could do it - see my magnet example above. The sideways pull is equal on all sides, but the slightest movement away from dead centre and the magnet quickly finds it's way to one side of the tube.
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by:HippyWarlock
ID: 10649258
OK - Let's be totally pragmatic about this then.

You cannot drill a hole through the centre of the Earth for the following reasons-

1) The material you make the drill out of will melt.
2) If you attempt to cool the drill it will melt
3) If you drill from both sides at once - both drills will melt
4) any system that would enable you to do it would be cost-prohibitive
5) Tectonic movements and huge lava tidal forces would soon bend your tube
6) Your tube would melt
7) Your apples would melt
8) Your funding would dry up

This is my puerile way of saying 'OK your right'

Peace
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by:R_Rajesh
ID: 10649317
:)
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ID: 10649622
Who's right, me ? - don't really know unless it's tested.

It has just occurred to me that most people have tried to dig their way to the other side of the world at least once (when they were young), and that all these holes have probably gone to waste and been filled in (by Dad who didn't want a hole in the lawn). What it needs is for someone to co-ordinate all these kids efforts and get them to dig in the same place. This way, over the course of say a couple of hundred years or so they could communally achieve quite a sizeable hole - maybe even deep enough to test the apple theory...
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by:ozo
ID: 10649677
> Why will they rest against one side of the tube? Surely if the tube is central then all sides will exert an equal pull?
Unless the tube is allpng the north-south axis the Coriolis effect will cause the path of a falling apple to curve relative to the tube. (because the tube would actually be rotating)
Once they come to rest they will rest against the side of the tube only if the tube or the distribution of mass around it is not central.
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I'm unconvinced about the rest position ozo,

If the apples are laying against one side of the tube, and all the distribution of mass is equal around a point in the centre of the tube (the apples are away from this point by an amount equal to the radius of the tube). Why should the apples drift away from their rest-point? They are closer to the mass on their near side and so this pull should overcome a slightly weaker attraction from the opposite side of the tube.
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by:ozo
ID: 10650545
This difference will be infinitesimal unless the tube diameter is large
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by:gillit
gillit earned 20 total points
ID: 10650680
If Earth was a static round object with uniform mass (except where the tube is, but for simplicity it is too insignificant to affect the whole mass) in an ideal state, the apple should oscillate from one end to the other end with non-constant acceleration (given that the apple wouldn't explode since gravity will be pulling it from all sides).
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by:HippyWarlock
ID: 10650725
The coriolis effect will be negligable unless the apples are large.

The earth is pear shaped and wobbles in its orbit, what does that do to the apples sense of identity?

Peace
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by:HeathAdams
HeathAdams earned 20 total points
ID: 10652775
The mole people in the center of the earth would eat the apples destroying the experiment.  Mole people love apples!  Better do it with hard-boiled eggs.  Mole people hate hard-boiled eggs!
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by:delphized
delphized earned 20 total points
ID: 10652871
the apples will smash and generate a toric surface well known as a doughnut! :-)) he he
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by:HippyWarlock
ID: 10654075
Urgh! Mole people!

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ID: 10655872
>the apple wouldn't explode since gravity will be pulling it from all sides

no more than they are torn in half whilst hanging from the tree, the full force of earths gravity is trying to pull it off it's stalk, whereas when it's bobbing about in the centre of the tube (or lying against one side of it) there is half this force acting in each direction so the net result is about the same.
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by:ozo
ID: 10655990
much less than whilst hanging from tree
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by:HippyWarlock
ID: 10656190
Yggrdrssly What is that tree?

The aplle couldn't rip apart as the force is equal - it will simply get bigger, as will the mole ppl.


Dear R Rajesh - please choose mathematically and geometrically pure and simple objects next time

:-)

Peace
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by:Urhixidur
Urhixidur earned 70 total points
ID: 10661951
Air resistance inside the tube does complicate matters tremendously, and things get worse if you assume the air in the tube has the same temperature profile as the Earth itself.  So, we'll assume the tube is perfectly straight and empty, and lies along the Earth's rotation axis (to avoid Coriolis effects).  We'll also assume the Earth is radially uniform (that is to say, all of the points lying at a given radius have the same physical pertinent characteristics --namely, density).

It is well-known (to astrophysics students) that a spherical shell of uniform matter exerts precisely no gravity anywhere inside itself.  Not so with an ellipsoidal shell, but since we're sticking to the short axis (the Earth being slightly oblate), the result holds.  We have zero gravity at the centre.  Off-axis, the Earth's equatorial bulge is indeed going to make the central position unstable, but the magnitude of the effect is really small.

So, the apples will speed up and smash into each other, making apple sauce.  The sauce will eventually settle in a torus in the Earth's equatorial plane, against the sides of the tube (assuming those sides to be frictionless, of course).

> HippyWarlock
> I wonder if you drilled a hole in a black hole, what the apple would do under the extreme tugging of the gravities

   You cannot drill a hole through a black hole, as there is no space to drill through.  A black hole is a hole in space-time: it has no actual inside, the universe stopping at its surface.

> gillit
> [...] given that the apple wouldn't explode since gravity will be pulling it from all sides.

   The gravity gradient across the apple's diametre is absolutely insignificant.  It is actually "much larger" at the Earth's surface than at its centre.
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by:HippyWarlock
ID: 10664137
>You cannot drill a hole through a black hole, as there is no space to drill through.  A >black hole is a hole in space-time: it has no actual inside, the universe stopping at its >surface.

What? surely with a really powerfull Black n Decker?

If the UniV stops at its surface, how come the stuff falling into it doesnt? Only the light stops at its surface

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by:Urhixidur
ID: 10667278
>  how come the stuff falling into it doesnt?

   But it does.  For a distant observer (i.e. in our reference frame), anything that falls toward the horizon takes infinite time to reach it.  So, in that sense, nothing has yet fallen in or ever will.  In the frame of the falling object, it does take only a finite time to reach and cross the horizon, but what of the rest of the Universe in the mean time?  Either it'll have collapsed back into a primeval fireball --so everything catches up to the infalling stuff before it crosses the horizon-- or it'll have stretched out to heat death --in which case the black hole will have evaporated through Hawking radiation before the infalling stuff can cross the horizon.  You just can't do it!
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by:gillit
ID: 10668046
HippyWarlock, Urhixidur

You're right. I should have put more thought into it. An exploding apple was such an interesting thought when it popped into my head.
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by:info_expert
info_expert earned 20 total points
ID: 10670911
Ok, Here is the answer.

Apples would certainly oscilate always if there was no air in pipe. but the gravity of the earth is also related to its mass. while apple is moving in pipe , it will happen to pass throught the parts of pipe where the mass of earth will vary , so it is a strong possiblity for the apple to be dragged in resistence with the pipe, or its vibration may not be uniform.

Got??????????
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by:Urhixidur
ID: 10673775
> Apples would certainly osclilate always if there was no air in pipe. but the gravity of the earth is also related to its
> mass. while apple is moving in pipe , it will happen to pass throught the parts of pipe where the mass of earth will
> vary , so it is a strong possiblity for the apple to be dragged in resistence with the pipe, or its vibration may not be
> uniform.

   I'm not sure I understand your comment completely, so here goes anyway.   ;-)

   If you grant that the Earth's oblateness is going to be a rather small disturbation factor (you can treat the Earth as a sphere plus a toroidal "equatorial ring" for an excellent approximation), and take also for granted that the Earth's density, at any point within its interior, is a pure function of radius (also an excellent approximation, as pockets of lighter or denser matter are going to be weak disturbances), then:
   At any point within our polar-axis vacuum pipe, gravity will be strictly towards the centre, and vary smoothly with radius.  This follows from the well-known effects that a) spherical shells of mass exert no gravity inside them, and b) spherically symmetrical masses behave as point masses on their outsides (as far as gravity is concerned).  The basic curve will be g = ((4/3) G pi rho r), where G is the universal constant of gravitation, pi is the ratio of a circle's circumference to its radius, rho is the average density of the Earth, and r is our radius variable.  Now, Earth is actually denser towards its centre than near its surface, so the actual plot of g will start off (at r zero) with a steeper slope (because rho will be larger for that value of r) that will progressively weaken (bend down) until it crosses the original straight line at r = R, the Earth's average surface radius.
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by:ozo
ID: 10673809
Also, lunar tides will exert a gentle pull on the apple away from the moon.
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by:R_Rajesh
ID: 10752223
Sorry for the delay guys, was caught up in negotiations with the mole people (to make them stay away from my apples :)

Thanks,
Rajesh
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by:Kunfufaresi
ID: 10927190
Hello,

In my theory Gravity is largely miss interpreted, the idea action at a distance is paranormal and ghostish. You can not pull or push something with your thoughs unless telekinetics is real. Even if it is real :) it would still need some rope or stick to connect the two items together.

Now although ether theory was found not to be true 100 years ago or so, the experiment was wrong not the theory. Ether exists in all space, it is a moving particle like water air, light or em radiation travels in this substance, thats why it has wave pattern, again ether clears the rubbish that light travels in vacum, as a wave, but also as a particle, come on everyone wants to make physics look so difficult no kid will fall for it with all this unexplained phonomenas being tried to explained with a zillion teories that even common logic can not resist questioning. Why speed of light is constat? a particle with constant speed, but differen wave length I mean its like kid asks why earth is round and you answer God only had round shapes to fill in with mud left, he had used all the rest for the astroids.. ok bad analogy :)

Back to gravity, ether is everywhere, it flows and it can pass through matter, matter is like a fishingnet in the water, or air, air and water molecules hit it but since the hit ration is so small its obviously hard to detect. Also they come from all sides i mean like in water, you are aware of the pressure only if you compare it with air, but we can not compare ether presure with anything else.

But when masses get bigger the obvous happens ether particles entring from the left side slows down somewhat during their voyage through the earth. The ether particles coming in oposite direction on the other hand wont be slowed down. Think of ether particles comes from the sky push you down, ether from the earth that have sloved down on the other hand push you up, the result is downward push is stronger. Gravity is a pushing force not a pulling force, and this makes sense even to a 7 year old kid.

Science is the reflection of the mind maxwell said, and our minds are smooth and clear, until Freud its all his fault that physics went this way.. not to mention art and music :)

The ether theory also can easliy tell you why tidal waves occure, the area belove the moon gets less ether pressure as the ether particles now have traveled through the moon and sloved down somewhat, this creates an inbalance with the rest of the globe, the matter on the surface bellow the moon gets an upwards push equal to the rest of the globe, but the downward push has weakened somewhat due to the moon, and the result is material seems to be pulled by the moon, but it is only the forces pushing at both sides that changed.

I've created software simulation to simulate the whole solar system based on Ether theory, and it gives the exact same numbers as real life, it also helps us understand light much better, i'm working on magnetic theory now, they really are the only thing i fear could injur the theory, but again ether is obviously being pushed faster, and low/high pressure points are occuring.

Anyway back to apple question :P the apples would both stand still, actually the air on both sides of this tunel would quickly vanish to be filled in by more air, but what you have done would be pasically creating a hole in earths atmosfer all air could disapear from the surface of the earth. The apples would stand still like they do in space, maybe the angular momentum would let them go sideways as they are no longer influenced by gravity.

Actually except the problem with air rushing out/upwards you would have the perfect anti-gravity launching pad, in this area where gravitational weight would be ZERO! but the air would actually lift your space ship, letting you in space without the burden of engines.

Glad you read all of this, hopes this answer your original question

Kunfu Faresi
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