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Old Jul 25th 2017, 04:20 PM   #1
wad
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Gravity

If there is only one mass in the universe, a sphere with mass of 100 kg with a radius of 1 meter, what is the pulling force of gravity emanating from the sphere at 100 meters form the center of the sphere?
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Old Jul 25th 2017, 05:32 PM   #2
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Is this classical newtonian gravity or einsteinian gravity?
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Old Jul 25th 2017, 09:41 PM   #3
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Originally Posted by wad View Post
If there is only one mass in the universe, a sphere with mass of 100 kg with a radius of 1 meter, what is the pulling force of gravity emanating from the sphere at 100 meters form the center of the sphere?
Zero

Since this is the only mass in the universe there is nothing else for this sphere "to pull on".
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Old Jul 26th 2017, 05:04 PM   #4
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Originally Posted by wad View Post
If there is only one mass in the universe, a sphere with mass of 100 kg with a radius of 1 meter, what is the pulling force of gravity emanating from the sphere at 100 meters form the center of the sphere?
Perhaps the problem is supposed to refer to the gravitational potential? The formula would be
$\displaystyle U = - \frac{Gm}{r}$

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Old Jul 28th 2017, 06:26 AM   #5
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The specification of there only being a single mass in the universe invites all sorts of philosophical issues of the "does a tree falling in a forest make a noise if no-one hears it?" type.

Newtonian Gravity is the interaction between two masses.
Einsteinian Gravity is a measure of the space-time distortion caused by the mass.

However I suspect that all these kind of considerations are beyond the intended scope of the questioner,
and that Topsquarks answer is the one being sought...
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Old Jul 28th 2017, 06:42 AM   #6
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Originally Posted by Woody View Post
and that Topsquarks answer is the one being sought...
Really??!! and what is r in tp's equation?
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Old Jul 28th 2017, 09:46 AM   #7
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Originally Posted by oz93666 View Post
Really??!! and what is r in tp's equation?
$r = 100 \, m$
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Old Jul 28th 2017, 01:47 PM   #8
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Originally Posted by oz93666 View Post
Really??!! and what is r in tp's equation?
topsquark specified that this was Gravity potential which is a field. "r" in his equation is the distance from the given mass to a point in that field.
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Old Jul 29th 2017, 06:48 AM   #9
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Originally Posted by HallsofIvy View Post
topsquark specified that this was Gravity potential which is a field. "r" in his equation is the distance from the given mass to a point in that field.
Note: that's true only under special circumstances, such as outside a sphere with symmetric body with uniform mass density. Inside the sphere its different. its also different for a binary star system. The gravitational potential above a spiral galaxy is nothing like that. But the gravitational field of all bodies can be found by adding up all the point particles that its made of by integrating.
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