- #1
John Creighto
- 495
- 2
So I'm reading "Mathematical Physics" by Donald H.
Menzel, and I don't buy the following derivation from
section 2.12
The purpose of the derivation is derive the potential
energy at a point Po which is a distance Ro from the
center of a sphere of uniform density.
First they derive the amount an infinitesimal amount of
mass P(dv) located on the shell of the sphere contributes
to the potential energy at a point Po.
[tex]dV=-\frac{G \rho}{R}dxdydz[/tex]
The following expression is given to compute the
potential energy contribution of the infinitesimal piece
of mas dV at P(dV) on a point Po.
[tex]V=-G\rho \int_r^{r+dr}\int_{\theata
=0}^{\pi}\int_{\phi=0}^{2\pi}\frac{r^2sin^2( \theta ) dr
d \theata d \phi }{R}[/tex]
r is the distance from the center of the sphere to P(dV)
Ro is the distance from P to the center of the sphere
R is the distance from P(dV) to Po
A change in variables is derived by doing implicit
differentiation with r contant on the law of cosines:
[tex]R^2=R_o^2+r^2-2R_o r \ cos( \theta )[/tex]
which gives:
[tex]R \ dR = R_o r sin( \theta ) d \theta [/tex]
So far I agree but then they say that this implies:
[tex]V=-G\rho \int_r^{r+dr}\int_{R_o-r}^{R_o+r}\int_{0}^{2\pi}\frac{r}{R_o}drdR d \phi[/tex]
However, if I do the above subsitution I get an extra factor of [tex]sin( \theta )[/tex] left over.
Menzel, and I don't buy the following derivation from
section 2.12
The purpose of the derivation is derive the potential
energy at a point Po which is a distance Ro from the
center of a sphere of uniform density.
First they derive the amount an infinitesimal amount of
mass P(dv) located on the shell of the sphere contributes
to the potential energy at a point Po.
[tex]dV=-\frac{G \rho}{R}dxdydz[/tex]
The following expression is given to compute the
potential energy contribution of the infinitesimal piece
of mas dV at P(dV) on a point Po.
[tex]V=-G\rho \int_r^{r+dr}\int_{\theata
=0}^{\pi}\int_{\phi=0}^{2\pi}\frac{r^2sin^2( \theta ) dr
d \theata d \phi }{R}[/tex]
r is the distance from the center of the sphere to P(dV)
Ro is the distance from P to the center of the sphere
R is the distance from P(dV) to Po
A change in variables is derived by doing implicit
differentiation with r contant on the law of cosines:
[tex]R^2=R_o^2+r^2-2R_o r \ cos( \theta )[/tex]
which gives:
[tex]R \ dR = R_o r sin( \theta ) d \theta [/tex]
So far I agree but then they say that this implies:
[tex]V=-G\rho \int_r^{r+dr}\int_{R_o-r}^{R_o+r}\int_{0}^{2\pi}\frac{r}{R_o}drdR d \phi[/tex]
However, if I do the above subsitution I get an extra factor of [tex]sin( \theta )[/tex] left over.