# physics question

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#### Saiph

##### Guest
You have to consider where the mass is that's exerting a force on you.<br /><br />As you head further down into the earth, you now have it below you <i>and</i> above you.<br /><br />So the question is, does the matter above you exert a force differently than when it's below you?<br /><br />So, halfway down, do you feel the same gravity as when you're on the surface?<br /><br />The other point to possibly start at, is think of the symettry of the problem. The same amount of mass is on your right, as is on your left, distributed the same way as well...thus they should pull on you the same, producing what net force? to the left, to the right, or neither? <div class="Discussion_UserSignature"> <p align="center"><font color="#c0c0c0"><br /></font></p><p align="center"><font color="#999999"><em><font size="1">--------</font></em></font><font color="#999999"><em><font size="1">--------</font></em></font><font color="#999999"><em><font size="1">----</font></em></font><font color="#666699">SaiphMOD@gmail.com </font><font color="#999999"><em><font size="1">-------------------</font></em></font></p><p><font color="#999999"><em><font size="1">"This is my Timey Wimey Detector.  Goes "bing" when there's stuff.  It also fries eggs at 30 paces, wether you want it to or not actually.  I've learned to stay away from hens: It's not pretty when they blow" -- </font></em></font><font size="1" color="#999999">The Tenth Doctor, "Blink"</font></p> </div>

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#### dark_energy

##### Guest
Oh, I understand now. Since I am at the center, hypothetically, the earth is pulling me in every direction as there is an equal amount of mass of the earth all around me. I believe that's what you're trying to say. Thanks for your help. <div class="Discussion_UserSignature"> </div>

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#### norisk

##### Guest
Hey, so going toward the center of the Earth could turn out to be a great weight loss program!<br /><br />Just a reminder that mass is not the same thing as weight because unlike weight, mass is not dependent on the effects of "gravity."

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#### dark_energy

##### Guest
Err, yes. I already know that weight is simply the force of gravity acting on the mass. = But in order to figure out your weight on any planet, you need that equation, ie. w=(GmM)/d^2 <div class="Discussion_UserSignature"> </div>

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#### pocket_rocket

##### Guest
X*0=0 X/0=0<br />You cant divide a quantity into 0 parts

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#### centsworth_II

##### Guest
<font color="yellow">"You have to consider where the mass is that's exerting a force on you."</font><br /><br />Weightless yet crushed under extreme weight. Ironic. <div class="Discussion_UserSignature"> </div>

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#### najab

##### Guest
Your weight might be zero, but the pressure will be enough to kill you in an instant!

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#### dark_energy

##### Guest
<blockquote><font class="small">In reply to:</font><hr /><p>X*0=0 X/0=0 <br />You cant divide a quantity into 0 parts<p><hr /></p></p></blockquote><br /><br />0/x=0 <br />x/0=undefined, or infinite. <div class="Discussion_UserSignature"> </div>

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#### centsworth_II

##### Guest
It might have been more clear if I had said "Weightless yet crushed under extreme <b>pressure</b>." (But not as ironic)<img src="/images/icons/smile.gif" /><br /><br />If we suppose a hollow at the center of the Earth, and tolerable temperatures, a person might float weightlessly. But then we'd have to move the thread to "Phenomena".<img src="/images/icons/smile.gif" /><img src="/images/icons/smile.gif" /><br /> <div class="Discussion_UserSignature"> </div>

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#### dark_energy

##### Guest
If you don't mind being a human cube, go for it. <div class="Discussion_UserSignature"> </div>

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#### scull

##### Guest
Mind the question.<br /><br /><i>Weight</i> is not the same as <i>mass</i>.

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#### Saiph

##### Guest
correct-o-mundo. <br /><br />i just didn't want to come right out and say it. <div class="Discussion_UserSignature"> <p align="center"><font color="#c0c0c0"><br /></font></p><p align="center"><font color="#999999"><em><font size="1">--------</font></em></font><font color="#999999"><em><font size="1">--------</font></em></font><font color="#999999"><em><font size="1">----</font></em></font><font color="#666699">SaiphMOD@gmail.com </font><font color="#999999"><em><font size="1">-------------------</font></em></font></p><p><font color="#999999"><em><font size="1">"This is my Timey Wimey Detector.  Goes "bing" when there's stuff.  It also fries eggs at 30 paces, wether you want it to or not actually.  I've learned to stay away from hens: It's not pretty when they blow" -- </font></em></font><font size="1" color="#999999">The Tenth Doctor, "Blink"</font></p> </div>

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#### nexium

##### Guest
That formula does not give the correct answer when you are at or close to the surface of the planet. Below the surface you can get an approximation by subtracting the mass that is farther from the mass center than you are. At the mass center you would subtract all the Mass of the planet = Gm times 0 divided by 0 squared. Neil

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#### Saiph

##### Guest
To be compeletly accurate neil, that isn't an approximation, but a probable mathematical theorem. <div class="Discussion_UserSignature"> <p align="center"><font color="#c0c0c0"><br /></font></p><p align="center"><font color="#999999"><em><font size="1">--------</font></em></font><font color="#999999"><em><font size="1">--------</font></em></font><font color="#999999"><em><font size="1">----</font></em></font><font color="#666699">SaiphMOD@gmail.com </font><font color="#999999"><em><font size="1">-------------------</font></em></font></p><p><font color="#999999"><em><font size="1">"This is my Timey Wimey Detector.  Goes "bing" when there's stuff.  It also fries eggs at 30 paces, wether you want it to or not actually.  I've learned to stay away from hens: It's not pretty when they blow" -- </font></em></font><font size="1" color="#999999">The Tenth Doctor, "Blink"</font></p> </div>

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#### dragon04

##### Guest
However trivial, are we talking the geometrical center or gravitational center of the Earth?<br /><br /> <div class="Discussion_UserSignature"> <em>"2012.. Year of the Dragon!! Get on the Dragon Wagon!".</em> </div>

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#### derekmcd

##### Guest
If you had an equal amount of Earth's mass (density) in every direction, wouldn't you still feel the gravitational pull of either the moon or the sun? For that matter, is there anywhere in the universe where you can have absolute weighlessness (not counting freefall which only gives you the sensation)? <div class="Discussion_UserSignature"> <div> </div><br /><div><span style="color:#0000ff" class="Apple-style-span">"If something's hard to do, then it's not worth doing." - Homer Simpson</span></div> </div>

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#### nexium

##### Guest
It appears there are points here and there where the net gravity is zero, but these points move as the moon and Sun change relative positions, likely only a few millimeters near the Earth's center.<br />I agree the zero g point is also displaced slightly from the geo center, because the Earth's RMS density is minutely differant on each radius. RMS = root mean square. Neil

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