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2025-07-07T21:48:37Z

Joel2: Created page with " =Force Exerted by a Uniform-Density Shell or Sphere= ==Tohline 1982== ===General Derivation from Notes Dated 29 November 1982=== If the force per unit mass exerted at the position, $~\vec{r}$ , from a single point mass, $~m$ , is given by,
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2024-07-11T17:37:05Z
Created page with "  =Force Exerted by a Uniform-Density Shell or Sphere=  ==Tohline 1982== ===General Derivation from Notes Dated 29 November 1982=== If the force per unit mass exerted at the position, <math>~\vec{r}</math>, from a single point mass, <math>~m</math>, is given by, <div align="center"> <table border="0" cellpadding="5" align="center"> <..."
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=Force Exerted by a Uniform-Density Shell or Sphere=


==Tohline 1982==

===General Derivation from Notes Dated 29 November 1982===
If the force per unit mass exerted at the position, <math>~\vec{r}</math>, from a single point mass, <math>~m</math>, is given by,
<div align="center">
<table border="0" cellpadding="5" align="center">

<tr>
<td align="right">
<math>~\vec{F}</math>
</td>
<td align="center">
<math>~=</math>
</td>
<td align="left">
<math>~- \biggl( \frac{G^'m}{r} \biggr) \frac{\vec{r}}{r} \, ,</math>
</td>
</tr>
</table>
</div>

then the force per unit mass exerted at <math>~\vec{x}</math> by a continuous mass distribution, whose mass density is defined by the function <math>~\rho(\vec{x}^')</math>, is,
<div align="center">
<table border="0" cellpadding="5" align="center">

<tr>
<td align="right">
<math>~\vec{F}(\vec{x})</math>
</td>
<td align="center">
<math>~=</math>
</td>
<td align="left">
<math>~- \int G^' \rho(\vec{x}^') \biggl[ \frac{\vec{x}^' - \vec{x}}{| \vec{x}^' - \vec{x} |^2} \biggr] d^3x^' \, .</math>
</td>
</tr>
</table>
</div>
This central force can also be expressed in terms of the gradient of a scalar potential, <math>~\Phi(\vec{x})</math>, specifically,
<div align="center">
<table border="0" cellpadding="5" align="center">

<tr>
<td align="right">
<math>~\vec{F}(\vec{x})</math>
</td>
<td align="center">
<math>~=</math>
</td>
<td align="left">
<math>~- \vec\nabla\Phi(\vec{x}) \, ,</math>
</td>
</tr>
</table>
</div>
where,
<div align="center">
<table border="0" cellpadding="5" align="center">

<tr>
<td align="right">
<math>~\Phi(\vec{x}) </math>
</td>
<td align="center">
<math>~=</math>
</td>
<td align="left">
<math>~ \int G^' \rho(\vec{x}^') \ln | \vec{x}^' - \vec{x} | d^3x^' \, .</math>
</td>
</tr>
</table>
</div>

For a spherically symmetric mass distribution, <math>~\rho(r^')</math>, the magnitude of the force that is directed along the radial vector, <math>~\vec{r}^'</math>, and measured from the center of the mass distribution can be expressed as the following single integral over <math>~r^'</math>:
<div align="center">
<table border="0" cellpadding="5" align="center">

<tr>
<td align="right">
<math>~F(r) \equiv \vec{F}\cdot \frac{\vec{r}}{r} </math>
</td>
<td align="center">
<math>~=</math>
</td>
<td align="left">
<math>~ -2\pi G^' \int\limits_{R_1}^{R_2} \rho(r^') (r^')^2
\biggl[\frac{1}{r} + \frac{1}{2r^2 r^'} \biggl( r^2 - {r^'}^2 \biggr) \ln\biggl( \frac{r^' + r}{|r^' - r|} \biggr) \biggr] dr^'
\, .</math>
</td>
</tr>
</table>
</div>
This integral can be completed analytically if <math>~\rho(r^') = \rho_0</math>, that is, for a uniform-density mass distribution. Independent of whether the limits of integration, <math>~R_1</math> and <math>~R_2</math>, are less than or greater than <math>~r</math>, the integral gives,
<div align="center">
<table border="0" cellpadding="5" align="center">

<tr>
<td align="right">
<math>~F(r) </math>
</td>
<td align="center">
<math>~=</math>
</td>
<td align="left">
<math>~
- \frac{3G^'}{8r} \biggl( \frac{4\pi}{3}\rho_0 \biggr) \biggl\{ \biggl( R_2^3 - R_1^3 \biggr) + r^2 \biggl(R_2 - R_1\biggr)
</math>
</td>
</tr>

<tr>
<td align="right">
 
</td>
<td align="center">
 
</td>
<td align="left">
<math>~ +
r^3 \biggl[ \frac{1}{2} + \frac{1}{2}\biggl( \frac{R_1}{r} \biggr)^4 - \biggl( \frac{R_1}{r} \biggr)^2\biggr]
\ln\biggl( \frac{R_1 + r}{|R_1 - r|} \biggr)
</math>
</td>
</tr>

<tr>
<td align="right">
 
</td>
<td align="center">
 
</td>
<td align="left">
<math>~ -
r^3 \biggl[ \frac{1}{2} + \frac{1}{2}\biggl( \frac{R_2}{r} \biggr)^4 - \biggl( \frac{R_2}{r} \biggr)^2\biggr]
\ln\biggl( \frac{R_2 + r}{|R_2 - r|} \biggr)
\biggr\} \, .</math>
</td>
</tr>
</table>
</div>

If the position, <math>~r</math>, is located outside of a uniform-density sphere, then <math>~R_1 = 0</math> and <math>~R_2 < r</math>, so the aggregate acceleration becomes,
<div align="center">
<table border="0" cellpadding="5" align="center">

<tr>
<td align="right">
<math>~F(r)_\mathrm{out} </math>
</td>
<td align="center">
<math>~=</math>
</td>
<td align="left">
<math>~
- \frac{3G^'}{8r} \biggl( \frac{4\pi}{3}\rho_0 \biggr) \biggl\{ R_2^3 + r^2 R_2 -
r^3 \biggl[ \frac{1}{2} + \frac{1}{2}\biggl( \frac{R_2}{r} \biggr)^4 - \biggl( \frac{R_2}{r} \biggr)^2\biggr]
\ln\biggl( \frac{r+R_2}{r- R_2} \biggr)
\biggr\}
</math>
</td>
</tr>

<tr>
<td align="right">
 
</td>
<td align="center">
<math>~=</math>
</td>
<td align="left">
<math>~
- \frac{G^' M(R_2)}{r} \biggl\{
1 - 3 \sum_{n=1}^{\infty} \biggl( \frac{R_2}{r} \biggr)^{2n} \biggl[(2n-1)(2n+1)(2n+3) \biggr]^{-1}
\biggr\} \, ,
</math>
</td>
</tr>
</table>
</div>
where, <math>~M(R_2) \equiv 4\pi \rho_0 R_2^3/3</math>. If the position, <math>~r</math>, is located interior to a uniform-density ''shell'', then <math>~r < R_1 < R_2</math> and the aggregate acceleration is,
<div align="center">
<table border="0" cellpadding="5" align="center">

<tr>
<td align="right">
<math>~F(r)_\mathrm{shell} </math>
</td>
<td align="center">
<math>~=</math>
</td>
<td align="left">
<math>~
- \frac{4\pi}{3} G^' \rho_0 R_2 r \biggl\{1 - \frac{R_1}{R_2}
- 3 \sum_{n=1}^{\infty} \biggl[ \biggl( \frac{r}{R_2} \biggr)^{2n} - \frac{R_1}{R_2} \biggl( \frac{r}{R_1} \biggr)^{2n}\biggr] \biggl[(2n-1)(2n+1)(2n+3)
\biggr]^{-1} \biggr\} \, .
</math>
</td>
</tr>
</table>
</div>

If <math>~r</math> is inside a uniform-density sphere, then <math>~R_1 = 0</math> and <math>~ r < R_2</math>, so the aggregate acceleration is,
<div align="center">
<table border="0" cellpadding="5" align="center">

<tr>
<td align="right">
<math>~F(r)_\mathrm{in} </math>
</td>
<td align="center">
<math>~=</math>
</td>
<td align="left">
<math>~
- \frac{4\pi}{3} G^' \rho_0 R_2 r \biggl\{
1 - 3 \sum_{n=1}^{\infty} \biggl( \frac{r}{R_2} \biggr)^{2n} \biggl[(2n-1)(2n+1)(2n+3) \biggr]^{-1}
\biggr\} \, .
</math>
</td>
</tr>
</table>
</div>

===Limiting Cases===
Some limiting cases are of interest for the uniform sphere, ''i.e.,'' when <math>~R_1 = 0</math>. First, notice that (Gradshteyn & Ryzhik 1965, formula 0.141-2),
<div align="center">
<table border="0" cellpadding="5" align="center">

<tr>
<td align="right">
<math>~\sum_{n=1}^{\infty} \biggl[(2n-1)(2n+1)(2n+3) \biggr]^{-1}</math>
</td>
<td align="center">
<math>~=</math>
</td>
<td align="left">
<math>~ \frac{1}{12} \, .</math>
</td>
</tr>
</table>
</div>

<font color="maroon" face="Arial">'''Sitting on the Surface:'''</font> Therefore, when <math>~r = R_2</math> — that is, ''on'' the surface of the uniform-density sphere,
<div align="center">
<table border="0" cellpadding="5" align="center">

<tr>
<td align="right">
<math>~F</math>
</td>
<td align="center">
<math>~=</math>
</td>
<td align="left">
<math>~ - \frac{3 G^' M(R_2)}{4R_2} \, .</math>
</td>
</tr>
</table>
</div>
So the force acts as though the mass is all concentrated at a point, ''not'' at the center of the sphere, but at a distance <math>~4/3</math> of the sphere's radius away.

<font color="maroon" face="Arial">'''Well Inside the Surface:'''</font> When <math>~r \ll R_2</math>,
<div align="center">
<table border="0" cellpadding="5" align="center">

<tr>
<td align="right">
<math>~F(r)_\mathrm{in}</math>
</td>
<td align="center">
<math>~\approx</math>
</td>
<td align="left">
<math>~ - \frac{G^' M(R_2)}{R_2} \biggl( \frac{r}{R_2} \biggr) \, ,</math>
</td>
</tr>
</table>
</div>
that is, the acceleration grows linearly with <math>~r</math>, as in any harmonic potential.

<font color="maroon" face="Arial">'''Well Outside the Sphere:'''</font> When <math>~r \gg R_2</math>,
<div align="center">
<table border="0" cellpadding="5" align="center">

<tr>
<td align="right">
<math>~F(r)_\mathrm{out}</math>
</td>
<td align="center">
<math>~\approx</math>
</td>
<td align="left">
<math>~ - \frac{G^' M(R_2)}{r} \, ,</math>
</td>
</tr>
</table>
</div>
which is in line with the adopted point-mass specification.

=See Also=

* [http://adsabs.harvard.edu/abs/1963MNRAS.127...21F Finzi (1963)] — ''On the Validity of Newton's Law at a Long Distance''
* [http://www.phys.lsu.edu/~tohline/TinsleyNotes1978.pdf Notes from Beatrice Tinsley dated July 3, 1978]
* [http://adsabs.harvard.edu/abs/1983IAUS..100..205T Stabilizing a Cold Disk with a 1/r Force Law]
* [http://www.phys.lsu.edu/~tohline/vita/Tohline.C5.pdf Does Gravity Exhibit a 1/r Force on the Scale of Galaxies?]
* [http://adsabs.harvard.edu/abs/1987ApJ...313....1K Kuhn & Kruglyak (1987)] — ''Non-Newtonian forces and the invisible mass problem''
* [http://arxiv.org/abs/1404.0531 Sanders (2014)] — ''A Historical Perspective on Modified Newtonian Dynamics''

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DarkMatter/UniformSphere - Revision history

Joel2: /* See Also */